If you loose access to all the devices with an active login, you're done. If your phone number expired and gets recycled by your isp, someone might use it in future (and that means they can login to your account if you don't have the 2SV set up - and if you have, they can ask for the account deletion after 7 days) Though, it's good to have your account associated to an active phone number you own because: I have an account that is using a phone number that expired two years ago, I just have to make sure to always have at least one valid Telegram session where to receive the verification code, in case I need to login to another device. You don't really need the phone number to be active to use Telegram. A verification code will be sent to the new number, you're not required to have access to the old phone number to complete the process

Open Telegram's settings and tap on your old phone number: Telegram will show you the option to change it. Login on another Telegram app (like Telegram Desktop), just to make sure to have another active session in case your phone falls into a river or explodes or something Enable two-step verification from the privacy and security settings, just in case your now-gone phone number gets reassigned to another person and that person uses it to login to Telegram

Telegram login with mobile number