Whats the point of that? You dont need to exchange phone numbers for telegram just the @username and only one side needs to know the others username.
And once you have a chat with someone both can share their own contact directly in the chat with 2 clicks and add it with 2 clicks as well.
(which is still rather useless because there is no real benefit from adding someone as contact. But I guess if you want to store number then this is easy)
You're thinking like a technically enlightened person -- if not an engineer -- who prioritizes efficiency and control.
You're not thinking like a "normie" goal-oriented user, who doesn't care about understanding the system, and for whom the shortest path to achieving their goal generally passes through saying "sure, whatever" to any requests the app makes.
You put the @username (the @ is optional) in the search field then click on the user and send a message. Once a message is send the chat will obviously stay in the chat list. Same for public groups/channels.
Alternatively you can share/click a link with the format t.me/username to skip the search part.