On telegram, if you add contacts from your phone number, it keeps the phone number. BUT. If you add people by screen name it doesn’t give you the phone number. So I can post @gb819 right here and nobody can get my number from it. but anyone could contact me through it. You run a search with the @ sign to search for people without the phone numbers in a “secret chat.” It’s if you aren’t on your phone, otherwise it’s an app.
Telegram also allows you to make large groups where everyone talks to each other.