Since I heard that Telegram stores all the chats including messages, pictures, audio & video material on their servers (cloud) I would like to know how can I delete all of my chats from my phone and also from their servers?
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See the FAQ: telegram.org/faq
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 and contacts. All groups and channels that you've created are orphaned and left without a creator but admins retain their rights.
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.
Everything you delete is deleted forever.