Note that this doesn't protect against impersonation. I can just compute the argon2 of "Alice" + "555-4939" and publish it in my profile.

And then you'll contact me and I'll pretend I'm your friend Alice and hilarity ensues or something more sinister.
 

 The Big 4 Evil Messengers have phone number verification for that reason. You'd have to manually verify Alice by contacting her via said Evil Messenger or SMS and verify that this is indeed who you think it is.