Comment by godelski

That is correct. I should have been clearer in my distinction. You can run your own server but that server won't connect to the official Signal network. You're completely fine to run your own[0]. FWIW I've seen other software roll their own servers and use the Signal protocol. I mean WhatsApp uses the Signal protocol but I think they've diverged a lot since.

[0] There's always talk about the big deal breaker for Signal being that it isn't federated. So I've always wondered why this passion isn't used to generate a federated Signal network and is more focused on Matrix (who only recently started being E2EE). I don't know how these things work, I'm not that kind of programmer, but I can't see why you couldn't modify the server code to work in a federated fashion and edit the app code to be able to connect to both? I'm actually interested to know why if someone actually has an answer.