As for telegram, I know that it is required for you to give your phone number in order to register. Does the bot owner know it or any other private information about you (like email address)? I've made a telegram, but I'm a newbie.
I think your number is safe and not revealed. Even on your telegram setting, you can be able to disable your phone number to be revealed to only your contacts, or not to be revealed to no one, if it can be revealed to bot without warning, it should have been on the setting.
Some bots need extra data from the user to work properly. For example, knowing the user's location helps provide more relevant geo-specific results. The user's phone number can be very useful for integrations with other services, like banks, etc.
Bots can ask a user for their location and phone number using special buttons. Note that both phone number and location request buttons will only work in private chats.
So, I think even if a bot will need access to your phone number, it will request for it and you will need to grant it access before it can be able to.