> Signal currently requires phone numbers for all its users. It does this not because Signal wants to collect contact information for its users, but rather because Signal is allergic to it: using phone numbers means Signal can piggyback on the contact lists users already have, rather than storing those lists on its servers.

But that's wrong? Signal totally has to build a global phone number database for the system to work, otherwise discovery wouldn't work. It doesn't matter if it hashes those numbers before, since the total range of phone numbers can be bruteforced with ease. Signal is a horrible examle for anonymous private messaging.

Serious alternatives are xmpp and matrix.