1) First of all open the Telegram app on your smartphone.
2) After the Telegram app is open, on the left side you have to click on the three dot menu.
3) After clicking on the three dot menu, click on Settings.
4) After clicking on Settings, tap on Privacy and Security.
5) In the privacy and security section you will see many options in the privacy section and one of these options is the phone number, you have to tap on it.
6) As soon as you tap on the phone number, you will see three options, Everybody, My Contacts and Nobody.
If you want to hide your phone number from all users, tap on Nobody, if you want the number which is saved in your contact list, only those people will be able to see your number, then you have to tap on My Contacts.