Very good question! We don't actually need or have any way to view/save/call your phone #.  

This is a Discord Verification setting.  Servers can set the Verification levels of the server to different settings.  This is particularly useful in preventing bots and spam accounts from mass-joining the server and ruining the experience for our community since we have a never expiring instant invite link posted.

If you don't wish to provide your phone number you can also setup a free Google Voice Account  and use the phone number from it as well.  The main thing is that it is an account verification.

Hopefully, that helps explain it.

Thanks a bunch this is very helpful. 😁 I will try the Google Voice Account thing. 👍