Advanced Telegram FAQ
Q: What is Telegram? What do I do here?
Telegram is a messaging app with a focus on speed and security, it’s super-fast, simple and free. You can use Telegram on all your devices at the same time — your messages sync seamlessly across any number of your phones, tablets or computers.
With Telegram, you can send messages, photos, videos and files of any type (doc, zip, mp3, etc), as well as create groups for up to 20,000 people or channels for broadcasting to unlimited audiences. You can write to your phone contacts and find people by their usernames. As a result, Telegram is like SMS and email combined — and can take care of all your personal or business messaging needs. In addition to this, we support end-to-end encrypted voice calls.
Q: How is Telegram different from WhatsApp?
Unlike WhatsApp, Telegram is a cloud-based messenger with seamless sync. As a result, you can access your messages from several devices at once, including tablets and computers, and share an unlimited number of photos, videos and files (doc, zip, mp3, etc.) of up to 1,5 GB each. And if you don’t want to store all that data on your device, you can always keep it in the cloud.
Thanks to our multi-data center infrastructure and encryption, Telegram is faster and way more secure. On top of that, Telegram is free and will stay free — no ads, no subscription fees, forever.
Our API is open, and we welcome developers to create their own Telegram apps. We also have a Bot API, a platform for developers that allows anyone to easily build specialized tools for Telegram, integrate any services, and even accept payments from users around the world.
And that‘s just the tip of the iceberg. Don’t forget to check out this paragraph for even more exclusive stuff.
Q: How old is Telegram?
Telegram for iOS was launched on August 14, 2013. The alpha version of Telegram for Android officially launched on October 20, 2013. More and more Telegram clients appear, built by independent developers using Telegram’s open platform.
Q: Which devices can I use?
You can use Telegram on smartphones, tablets, and even computers. We have apps for iOS (6 and above), Android (4.1 and up) and Windows Phone. You can also use Telegram’s web version or install one of our desktop apps for Windows, OSX, and Linux.
You can log in to Telegram from as many of your devices as you like — all at the same time. Just use your main mobile phone number to log in everywhere.
Our API is open for developers, should you want to build your own applications for other platforms.
Q: Who can I write to?
You can write to people who are in your phone contacts and have Telegram. Another way of contacting people is to type their Telegam username into the search field.
You can set a public username for your Telegram account too. Other people will be able to search and find you by that username — and send messages to you even if they don’t know your number. You can learn more about usernames here.
Q: How do I know who in my contacts has Telegram?
Your contacts, who have Telegram, are shown at the top of your Contacts. They also have pictures.
Q: How do I invite my friends?
iOS: The basic invitations are simple SMS messages. They will be charged as standard outgoing SMS by your carrier (unless sent via iMessage). Naturally, you have other options to bring your friends here. Try sending them a download link via any other messaging service: email, Facebook, WhatsApp, an actual telegram — you name it. The link: https://telegram.org/dl/
Android: Open the app menu (swipe right in chats list) — Invite Friends. Then choose an application via which you would like to send out invitations.
Windows Phone: Go to Contacts — Invite Friends. Then choose an application via which you would like to send out invitations.
You can give your friends a t.me link with your username so that they can easily find you on Telegram even if they don’t have your phone number.
Q: What do the green checks mean?
One check — message delivered to the Telegram cloud and your friend has been notified if he allows notifications.
Two checks — message read (your friend opened Telegram and opened the conversation with the message).
We don‘t have a ’delivered to device’ status for messages because Telegram can run on as many devices as you want. So which particular one would that check mean?
Q: Can I hide my ‘last seen’ time?
You can choose who sees this info in Privacy and Security settings.
Remember that you won‘t see Last Seen timestamps for people with whom you don’t share your own. You will, however, see an approximate last seen value. This keeps stalkers away but makes it possible to understand whether a person is reachable over Telegram. There are four possible approximate values:
Last seen recently — covers anything between 1 second and 2-3 days
Last seen within a week — between 2-3 and seven days
Last seen within a month — between 6-7 days and a month
Last seen a long time ago — more than a month (this is also always shown to blocked users)
Q: Who can see me ‘online’?
The last seen rules apply to your online status as well. People can only see you online if you’re sharing your last seen status with them. There is one exception to this: people will be able to see you online for a brief period when you send them a message in a one-on-one chat or in a group where you both are members.
Q: Can I make calls via Telegram?
Yes! Voice calls are currently available to users around the world.
Q: How can I use emoticons in Telegram on my iOS device?
We support emoji emoticons. Simply enable the Emoji keyboard in your iOS device‘s Settings (General – Keyboards – Add New Keyboard… – Emoji). Then switch to that keyboard whenever you’re out of words.
Groups, Supergroups, and Channels
Q: What makes Telegram groups cool?
Telegram groups can have up to 20,000 members each and are extremely powerful communication tools. Here are a few key features that make them stand out in the messaging world:
Edit your messages after posting, delete them so that they disappear for everyone.
Access your messages anytime, from any number of your mobile or desktop devices.
Find the message you’re looking for, even among millions. Filter by sender to make searching easier.
Replies, mentions, hashtags
Easily trace a conversation and keep communication efficient, no matter the group size.
Mute the group to get notifications only when people mention you or reply to your messages.
You can pin any message to be displayed at the top of the chat screen. All members will get a notification — even if they muted ordinary messages from your group.
Appoint administrators that can mass-delete messages, control membership, and pin important messages. Define their admin privileges with granular precision.
Send and receive files of any type, up to 1,5 GB in size each, access them instantly on your other devices.
Get a short link for your group and make it public, like t.me/publictestgroup. This way, anybody can view the group’s entire chat history and join to post messages.
Customization via bots
Create custom tools for any specific needs using our Bot API and Inline Bots.
Q: What’s the difference between groups, supergroups, and channels?
Telegram groups are ideal for sharing stuff with friends and family or collaboration in small teams, they can have up to 200 members and by default everyone can add new people and edit the name and group photo.
If your group grows to a very large community, you can upgrade it into a more centralized supergroup. Supergroups can have up to 20,000 members and have a unified history, so deleted messages will always disappear for everyone. You can also pin important messages to the top of the screen so that all members can see them, including those who have just joined.
Channels are a tool for broadcasting public messages to large audiences. In fact, a channel can have an unlimited number of subscribers. When you post in a channel, the message is signed with the channel’s name and photo and not your own.
Q: How do I create a group?
iOS: Start a new message (tap the icon in the top right corner in Chats). Then ‘New Group’.
Android: Tap the circular pencil icon in the chats list. Then ‘New Group’.
Windows Phone: Tap the “+” button on the bottom bar. Then ‘New Group’.
Q: Can I assign administrators?
Telegram groups are democratic by design. Everyone can invite new members and change the group‘s name and photo, which is ideal for small bands of friends or coworkers. For larger communities that need more administration, we’ve introduced a special restricted mode.
In the restricted mode, only administrators can add new people and change the name and photo of the group. Admins also have the power to remove other members from the group.
iOS: Go to Group Info (tap the photo in the top right corner on the group‘s chat screen) — Edit — Add Admins. Disable ’All Members Are Admins’ and appoint admins from the list.
Android: Go to Group Info (tap the name in the header) — … (in the top right corner) — Set Admins. Disable ‘All Members Are Admins’ and appoint admins from the list.
WP: Go to Group Info (tap the name in the header) — … (in the bottom right corner) — Set Admins. Disable ‘All Members Are Admins’ and appoint people from the list.
Q: How do I add more members? What’s an invite link?
You can add your contacts, or using search by username.
It is easy to migrate existing groups to Telegram by sending people an invite link. To create an invite link, go to Group Info — Add Member — Invite to Group via Link.
Anyone who has Telegram installed will be able to join your group by following this link. If you choose to revoke the link, it will stop working immediately.
Usernames and t.me
Q: What are usernames? How do I get one?
You can set up a public username on Telegram. It then becomes possible for other users to find you by that username — you will appear in contacts search under ‘global results’. Please note that people who find you will be able to send you messages, even if they don’t know your number. If you are not comfortable with this, we advise against setting up a username in Telegram.
You can set up a username in Settings and use the universal search box in the chats list to search for chats, messages, and usernames.
Q: How does t.me work?
Once you’ve set up a username, you can give people a t.me/username link. Opening that link on their phone will automatically fire up their Telegram app and open a chat with you. You can share username links with friends, write them on business cards or put them up on your website.
This way people can contact you on Telegram without knowing your phone number.
Q: What can I use as my username?
You can use a-z, 0-9 and underscores. Usernames are case-insensitive, but Telegram will store your capitalization preferences (e.g. Telegram and TeleGram is the same user). The username must be at least five characters long.
Q: Do I need a username?
You don‘t have to get one. Remember that Telegram usernames are public and choosing a username on Telegram makes it possible for people to find you in global search and send you messages even if they don’t have your number. If you are not comfortable with this, we advise against setting up a username.
Q: If someone finds me by username, messages and I reply — will they know my number?
No. Neither party will see another‘s phone number. This is similar to the case when you message a person who you’ve met in a Telegram group.
There is one exception to this: just as in the case of all Telegram messages, if you have somebody‘s number saved as a contact and you send them a message, your number also becomes visible to them. Just like with SMS. This happens regardless of how you open the conversation with that person — via the contacts menu, global search by username, t.me link, or from a group’s members page.
Q: How do I delete my username?
Go to Settings and save an empty username. This will remove your username; people will no longer be able to find you via search. This will not affect existing conversations.
Q: What do I do if my username is taken?
Telegram usernames are distributed on a first come — first serve basis.
We understand that certain usernames are part of an online identity for some of us. If your desired username is already taken, we will be happy to help you acquire it for your account or channel, provided that you have that same username on at least two of these services: Facebook, Twitter, Instagram.
Due to the fact that one account can register multiple bot and channel usernames, we reserve the right to recall usernames assigned to unused bots and channels, as well as openly squatted usernames.
Please contact our in-app Support if you have more questions (Settings — Ask a question). Notice that we rely on volunteers support in Telegram (so please be nice). Ping us on Twitter if you get no response within a day or so.
If you’re a developer, you may find our Bots FAQ more useful.
Q: How do I create a bot?
Creating Telegram bots is super-easy, but you will need at least some skills in computer programming. If you‘re sure you’re up to it, our Introduction for Developers is a good place to start.
Unfortunately, there are no out-of-the-box ways to create a working bot if you are not a developer. But we‘re sure you’ll soon find plenty of bots created by other people to play with.
Q: A bot is sending me messages, how do I make it stop?
If you don‘t want a bot to send you messages, feel free to block it – same as you would block a human user. Some Telegram clients have a ’Stop Bot‘ button right in the bot’s profile.
That said, most bot developers offer commands that silence the bot, check its /help for clues.
Q: Are bots safe?
Yes. Bots are no different from human users that you meet in groups for example. They can see your public name and profile pictures, and they can see messages you send to them, that’s it. They don’t see your phone number (unless you decide to give it to them yourself).
Naturally, any bot should be treated as a stranger — don’t give them your passwords, Telegram codes or bank account numbers, even if they ask nicely. Also, be careful when opening files sent by bots, same as you would deal with ordinary humans. Example: If a bot sent us a file called OpenMe.exe, we probably wouldn’t open it.
Q: If I add a bot to my group, can it read my messages?
Bots can work in two modes when you add them to groups. By default, bots only see messages that are meant for them. In this case, you‘ll see ’has no access to messages’ in the group members list next to the bot.
Some bots need more information to work, so developers may disable privacy mode. In this case, the bot will see all messages sent to the group, and you will see ‘has access to messages’ in the members list next to the bot.
Learn more about privacy mode for bots »
If your group contains very sensitive information, maybe it‘s better to avoid adding bots you don’t trust 100%.
Q: Do you have official bots?
Sorry, we don‘t have any official bots and don’t build them ourselves. We‘ve mentioned a few bots created by the platform’s beta testers in our blog. Maybe you’ll like them?
Q: Where can I find more bots?
There is no official store at the moment, so you‘ll have to ask your friends or search the web for now. We’re pretty sure you’ll find some bots to play with.
If you are an advanced user, you may find our FAQ for the Technically Inclined useful as well.
Q: How secure is Telegram?
Telegram is more secure than mass market messengers like WhatsApp and Line. We are based on the MTProto protocol (see description and advanced FAQ), built upon time-tested algorithms to make security compatible with high-speed delivery and reliability on weak connections. We are continuously working with the community to improve the security of our protocol and clients.
Q: What if I’m more paranoid than your regular user?
We’ve got you covered. Telegram’s special secret chats use end-to-end encryption, leave no trace on our servers, support self-destructing messages and don’t allow forwarding. On top of this, secret chats are not part of the Telegram cloud and can only be accessed on their devices of origin.
Q: So how do you encrypt data?
We support two layers of secure encryption. Server-client encryption is used in Cloud Chats (private and group chats), Secret Chats use an additional layer of client-client encryption. All data, regardless of type, is encrypted in the same way — be it text, media or files.
Our encryption is based on 256-bit symmetric AES encryption, 2048-bit RSA encryption, and Diffie–Hellman secure key exchange. You can find more info in the Advanced FAQ.
See also: Do you process data requests?
Q: Why should I trust you?
Telegram is open, anyone can check our source code, protocol and API, see how everything works and make an informed decision. In fact, we welcome security experts to audit our system and will appreciate any feedback (firstname.lastname@example.org). We offer bounties for successfully identified vulnerabilities.
On top of that, Telegram’s primary focus is not to bring a profit, so commercial interests will never interfere with our mission.
See also: articles about Telegram
Q: Do I need to trust Telegram for this to be secure?
When it comes to secret chats, you don‘t — just make sure that the visualized key of your secret chat matches the one in your friend’s secret chat settings. More about this below.
Q: What if my hacker friend says they could decipher Telegram messages?
Anyone who claims that Telegram messages can be deciphered is welcome to prove that claim in our competition and win $300,000. You can check out the Cracking Contest Description to learn more.
Comments on Telegram’s security outside the scope of the contest are welcome at email@example.com. We offer bounties for successfully identified vulnerabilities in our apps and protocol.
Q: Can Telegram protect me against everything?
Telegram can help when it comes to data transfer and secure communication. This means that all data (including media and files) that you send and receive via Telegram cannot be deciphered when intercepted by your ISP, network administrator or other third parties.
But please remember that we cannot protect you from your own mother if she takes your unlocked phone without a passcode. Or from your IT-department if they access your computer at work. Or from any other people that get physical or root access to your phones or computers running Telegram.
If you have reasons to worry about your personal security, we strongly recommend using only Secret Chats in official or at least verifiable open-source apps for sensitive information, preferably with a self-destruct timer. We also recommend enabling 2-Step Verification and setting up a h3 passcode to lock your app, you will find both options in Settings — Privacy and Security.
Q: How does 2-Step Verification work?
Logging in with an SMS code is an industry standard in messaging, but if you’re looking for more security or have reasons to doubt your mobile carrier or government, we recommend protecting your cloud chats with an additional password.
You can do this in Settings – Privacy and Security – 2-Step Verification. Once enabled, you will need both an SMS code and a password to log in. You can also set up a recovery email address that will help regain access, should you forget your password. If you do so, please remember that it’s important that the recovery email account is also protected with a h3 password and 2-Step Verification when possible.
Check this out for tips on creating a h3 password that is easy to remember.
Q: Why can jailbroken and rooted devices be dangerous?
Using a rooted or jailbroken device makes it easier for a potential attacker to gain full administrative control over your device — root access.
A user with root access can easily bypass security features built into the operating system, read process memory or access restricted areas, such as the internal storage. Once an attacker has root access, any efforts to mitigate threats become futile. No application can be called safe under these circumstances, no matter how h3 the encryption.
Q: How are secret chats different?
Secret chats are meant for people who want more secrecy than the average fella. All messages in secret chats use end-to-end encryption. This means only you and the recipient can read those messages — nobody else can decipher them, including us here at Telegram (more on this here). On top of this, Messages cannot be forwarded from secret chats. And when you delete messages on your side of the conversation, the app on the other side of the secret chat will be ordered to delete them as well.
You can order your messages, photos, videos and files to self-destruct in a set amount of time after they have been read or opened by the recipient. The message will then disappear from both your and your friend’s devices.
All secret chats in Telegram are device-specific and are not part of the Telegram cloud. This means you can only access messages in a secret chat from their device of origin. They are safe for as long as your device is safe in your pocket.
Q: How do I start a secret chat?
iOS: Start a new message (tap the icon in the top-right corner in Messages). Then ‘New secret chat’.
Android: Swipe right to open the menu, then ‘New secret chat’.
WP: Tap + in the chats list, then ‘New secret chat’.
Remember that Telegram secret chats are device-specific. If you start a secret chat with a friend on one of your devices, this chat will only be available on that device. If you log out, you will lose all your secret chats. You can create as many different secret chats with the same contact as you like.
Q: How do self-destructing messages work?
The Self-Destruct Timer is available for Secret Chats only. To set the timer, simply tap the clock icon (in the input field on iOS, top bar on Android, chat info page on WP), and then Set Self-Destruct Timer to the desired time limit. The clock starts ticking the moment the message is displayed on the recipient’s screen (gets two green checks). As soon as the time runs out, the message disappears from both devices.
Photos sent with short self-destruct timers (< 1 minute) can only be viewed while you’re holding your finger on them — and we will try to send a notification whenever a screenshot is taken.
Please note that the timer only applies to messages that were sent after the timer was set. It has no effect on earlier messages.
Q: Can I be certain that my conversation partner doesn’t take a screenshot?
Unfortunately, there is no bulletproof way of detecting screenshots on certain systems (most notably, some Android and Windows Phone devices). We will make every effort to alert you about screenshots taken in your Secret Chats, but it may still be possible to bypass such notifications and take screenshots silently. We advise to share sensitive information only with people you trust. After all, nobody can stop a person from taking a picture of their screen with a different device or an old school camera.
Q: What‘s this ’Encryption Key’ thing?
When a secret chat is created, the participating devices exchange encryption keys using the so-called Diffie-Hellman key exchange. After the secure end-to-end connection has been established, we generate a picture that visualizes the encryption key for your chat. You can then compare this image with the one your friend has — if the two images are the same, you can be sure that the secret chat is secure, and no man-in-the-middle attack can succeed.
Newer versions of Telegram apps will show a larger picture along with a textual representation of the key (this is not the key itself, of course!) when both participants are using an updated app.
Always compare visualizations using a channel that is known to be secure — it’s safest if you do this in person, in an offline meeting with the conversation partner.
Q: Why not just make all chats ‘secret’?
All Telegram messages are always securely encrypted. Messages in Secret Chats use client-client encryption, while Cloud Chats use client-server/server-client encryption and are stored encrypted in the Telegram Cloud (more here). This enables your cloud messages to be both secure and immediately accessible from any of your devices – even if you lose your device altogether.
The problem of restoring access to your chat history on a newly connected device (e.g. when you lose your phone) does not have an elegant solution in the end-to-end encryption paradigm. At the same time, reliable backups are an essential feature for any mass-market messenger. To solve this problem, some applications (like Whatsapp and Viber) allow unencrypted backups that put their users’ privacy at risk – even if they do not enable backups themselves. Other apps ignore the need for backups altogether and fade into oblivion before ever reaching a million users.
We opted for a third approach by offering two distinct types of chats. Telegram disables default system backups and provides all users with an integrated security-focused backup solution in the form of Cloud Chats. Meanwhile, the separate entity of Secret Chats gives you full control over the data you do not want to be stored.
This allows Telegram to be widely adopted in broad circles, not just by activists and dissidents, so that the simple fact of using Telegram does not mark users as targets for heightened surveillance in certain countries. We are convinced that the separation of conversations into Cloud and Secret chats represents the most secure solution currently possible for a massively popular messaging application.
Q: Why not open source everything?
All code will be released eventually. We started with the most useful parts — a well-documented API that allows developers to build new Telegram apps, and open source clients that can be verified by security specialists.
Q: Can I run Telegram using my own server?
Our architecture does not support federation yet. Telegram is a unified cloud service, so creating forks where two users might end up on two different Telegram clouds is unacceptable. To enable you to run your own Telegram server while retaining both speed and security is a task in itself. At the moment, we are undecided on whether or not Telegram should go in this direction.
Q: Can I use the Telegram API?
Yes. Developers for all platforms are welcome to use our protocol, API and even source code. Check out the Getting started section of the docs.
Don’t forget about our Bot API that lets you build cool stuff on our platform.
Sure. Check this out.
Q: Why do you have two apps in the Mac App store?
One is our native OSX app, the other is the OSX version of our multi-platform client— Telegram Desktop. Both apps are official. Both started out as unofficial applications by two different developers and vary in design and functionality.
Telegram Desktop is currently the more stable version. It is optimized for photo and document sharing, as well as sending long text messages.
The OSX app features native OSX design and supports secret chats.
Q: Can I translate Telegram?
Telegram is officially available in English, Spanish, German, Dutch, Italian, Arabic, Portuguese and Korean on most platforms.
We are gradually expanding the list of official languages built into the apps, but there are other options as well. Telegram apps support external language packs. The .xml files for the Android app and Localizable.strings files for iOS can be downloaded from this Transifex page. Of course, you can also change the language files in any way you like and thus create your own personal Telegram localization.
To apply an external language pack, send (or receive) the file via Telegram (attached as ‘file’), download it, then tap on it to see the menu (on Android: tap on … in the upper right corner of the doc bubble) then select ‘Apply localization file’. You will see a language selection menu with the language from the file as one of the options. Yes, this is quite tricky, we’ll simplify the process at some point in the future.
Q: Can I help?
Yes, we are always looking for volunteers to help us with user support. If you would be interested in answering questions about Telegram to users from your country, contact our auditions account.
Before you apply, please check out the Telegram Support Initiative.
Q: How do I log out?
Remember, that you can use Telegram on as many devices at the same time, as you like. If you log out, you don’t lose your cloud messages. However, you will lose all your Secret Chats and all messages inside them when you log out.
iOS: Go to Settings, then Edit, then Log out.
Android, Windows Phone: Go to Settings, then Log out.
Note that logging out doesn‘t trigger remote deletion of your secret chat messages on your partner’s device — to do that, choose ‘clear history’ first.
iOS: Swipe left on the secret chat, choose ‘Clear History’.
Android: Long tap on a chat, choose ‘Clear history’.
Q: How do I change my phone number?
You can change your number in Telegram and keep everything, including all your contacts, messages, and media from the Telegram cloud, as well as all your Secret Chats on all devices.
Important: Your mutual contacts (people in your contacts who also have your number) will get your new number added to your contact in their address book unless you have blocked them in Telegram. This makes changing numbers even easier.
To change your number, go to Settings, then tap on your phone number (just above the username), then ‘Change Number’. If you already have a different Telegram account on the target number, you’ll need to delete that account first.
Q: How do I delete my account?
If you would like to delete your account, you can do this on the deactivation page. Deleting your account permanently removes all your messages, groups and contacts. This action must be confirmed via your Telegram account and cannot be undone.
We recommend using a non-mobile browser for this process.
Note that you’ll receive the code via Telegram, not SMS.
Q: What happens if I delete my account?
As was just mentioned above, all your data will be flushed from our system: all messages, groups, and contacts associated with your account will be deleted. That said, your contacts will still be able to chat in the groups that you have created, and they will still have their copy of the messages you sent them. So if you want to send messages that can vanish without a trace, try using our self-destruct timer instead.
Termination of a Telegram account is irreversible. If you sign up again, you will appear as a new user and will not get your history, contacts or groups back. People, who have your phone number in their contacts, will be notified. The new user will be displayed as a separate conversation in their messages list and their conversation history with this new user will be empty.
Q: How does account self-destruction work?
Telegram is not a commercial organization, and we value our disk space greatly. If you stop using Telegram and do not log in for at least six months, your account will be deleted along with all messages, media, contacts and every other piece of data you store in the Telegram cloud. You can change the exact period after which your inactive account will self-destruct in Settings.
This feature was introduced on November 19, 2014. Since the default self-destruct setting for Telegram accounts is six months, accounts that have been last active before November 19, 2014, self-destructed on May 19, 2015.
Q: My phone was stolen. What do I do?
First of all, sorry about your phone. Unfortunately, the phone number is the only way for us to identify a Telegram user at the moment. We don‘t collect additional information about you, so whoever has the number, has the account. This means we can’t help you unless you have access either to the phone number or to Telegram itself on any of your devices.
I have access to Telegram on another device
Go to Telegram Settings — Privacy and Security and turn on Two-Step Verification. This way the phone number alone will not be enough to log in to your account.
Go to Settings — Privacy and Security — Active Sessions and terminate your Telegram session on the old device. Whoever has your phone will not be able to log in again, since they don’t know your password.
Contact your phone provider, so that they block your old SIM and issue a new one with your number.
If you decide to switch to a new phone number, don’t forget to go to Settings, tap on your phone number and change your Telegram number to the new one.
I don’t have access to Telegram on any other devices
First and foremost, you need to contact your phone provider, so that they block your old SIM and issue a new one with your number.
Wait till you receive your new SIM with the old number, log in to Telegram, then go to Settings — Privacy and Security — Active Sessions and terminate your Telegram session on the old device.
Removing sensitive data
Common thieves usually throw out the SIM card immediately (the phone is harder to locate this way), then wipe the devices and sell them, so there isn’t much risk for the data in case of regular petty theft. But if you have reasons to worry about the data on the device and are unable to log out the other device, it is best that you wipe it remotely. You can read more about it here: Apple iOS, Android. Unfortunately, this requires you to have prepared in advance for this scenario.
You can delete your Telegram account if you are logged in on at least one of your other devices (mobile or desktop). Note that inactive Telegram accounts self-destruct automatically after a period of time — 6 months being the default setting.