hdishs I don't trust the signal servers nor do I want to register with a phone number.
The whole point of e2e is you don't have to trust servers. Your messages/calls are encrypted and the keys are on the two devices communicating. As for phone numbers, nothing to dislike there. It goes to threat model. For lower threat model I really like the fact that Signal is pretty popular and contacts show who is using Signal so I can have e2e communication with them. I am already putting myself out there as a Signal user to my contacts, so I am not implying high threat model. Phone numbers are a truncated hash in a secure enclave on an encrypted Signal server. Could there be a side-channel secure enclave attack revealing your contacts on a centralized server? Sure. That's why I see Signal as secure communications, but otherwise not high threat model as contacts/groups are not so secure. Also not a fan of Signal using SGX for secure enclave, but again, it does not go to my lower threat model for my use of Signal. Now, at some point I might be using a high threat model where I want more anonymity to communicate with a person or two for a specific reason. Session is more appropriate for me. No phone number, decentralized onion routing system, IP hidden from ISP by default, essentially no metadata. Reason it is not my go-to privacy messenger is hardly anyone uses it and Signal serves my purposes with a lot of users for private communication. Signal was never meant for anonymous communication. It has always been meant for secure communication.
Also, if high threat model, don't get caught up too much on no phone number. You will still (in most cases) have a username attached to your identity. Anonymity is nowhere near as simple as not requiring PII during registration. So I pick a username for Session. That's a permanent connection between my Session identity and my real identity. If I choose another very unique username for another Session account, that username is also attached (anonymously at the time) to people I communicate with on Session and even other unique usernames are for contacts/cross contacts on other forms of communication like regular email, or e2e email, or a Cheo/XMPP username which is an email, VOIP, forums I belong to like this one with a unique username, bill and shopping usernames, etc. Even with all different unique usernames communicating with other people/websites, if you are high threat model and targeted, the NSA via OSINT and correlation will have a pretty good if not very good chance of identifying you - especially over time (and that's assuming you are not doxxed by a contact). Is a username on an e2e messenger better than a phone number for some anonymity? Sure.
I see Signal as having high threat model encryption, but not good for anonymity. That's the use case. Some people choose a more popular and light canvas bag for groceries. Others might choose a more sturdy but heavier burlap bag. Nobody is wrong with their personal choice. And, some people make well thought out personal choices and others not so much. That's life.