While you can hide your phone number, I don't think that is the default.

https://telegram.org/faq#q-who-can-see-my-phone-number

Doesn't your link say exactly what parent said?

Now that I think of it, my comment was unnecessary.

> your number is only visible to people who you've added to your address book as contacts

Means they already have my number.

There is still a tiny difference. I can pick a username and chat with people who have my phone number (without them knowing it is me). It's a very small difference.

Well not quite, since you might have had their number but they not yours. But if you just talk to someone on Telegram, they are not automatically added to your address book, and thus can't see it by default (as I understand it)