The server owners can’t see your phone number. Discord allows you to set a minimum verification level for users in your server. The lowest is simply having a verified email on their account. The highest is a verified email, a few minutes old (so no brand new accounts), been in the server for a few minutes, and a verified phone number. It’s just a bot prevention thing, because spammers/scammers are a big issue on large Discord servers. Especially early in Discord’s history, it was a big issue where bots would raid a server and just totally shut it down with spam links. So Discord started allowing server owners to set minimum verification levels before users could interact and send messages.