When the phone number is visible
- If you add someone as a phone contact and write them a message, the recipient will be able to see your number.
- If you contact a user via username, forwarded message or group, your number remains 'unknown' until it's shared.
- Once the number is shared, you will always see it — even if you delete it from your contacts list. No reason to hide it again.
- 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. If you want to hide your new number, you can either block these users or create a new account with this new number.
You meet a nice looking dude on the street, let's call him Marco (like Marco Polo). A stranger. You give him your Telegram username and wait for him to message you. Probably he won't message you (sorry about that), but let's just imagine he does. Will he see your number? Depends. If you have his number already saved then he will see your number (to keep it fair). The chance is close to zero since he is a stranger, why should he already have your phone number? Same goes for group chats. Will group members see your number? Only if you have their number or they have yours.
By the way 1x number exchanged = always exchanged. No way to hide it again (yep no pranks, sorry).