Phone numbers are every message board person's gripe with Signal, and it's a totally understandable gripe. But Signal doesn't demand a phone number because they want your personal information, but rather because they're allergic to it: to run a messaging system, you have to provide users with contact lists, and the way all other services do that is to store contact lists in serverside databases. That means that other messaging applications store the complete contact graph of the service --- literally a log of precisely who talks to whom --- in plaintext, in a single centralized location. Signal avoids doing that by piggybacking on the contact list you already have, and so doesn't have to store a contact graph at all. It's a UX sacrifice Signal makes in favor of superior privacy and security; UX sacrifices for security are basically Signal's brand.