The thing about phone numbers, though, is that you already have a lot of them in your contacts. Centralized apps can leverage that. Usernames are nice, but you

The thing about phone numbers, though, is that you already have a lot of them in your contacts. Centralized apps can leverage that. Usernames are nice, but you probably don't know whether your dentist uses Discord, or what is their username if they do. With Signal, however, you would probably notice (or be informed) when they sign up.