I already have a VoIP phone number through JMP and would be happy to use it for Signal, but I've read that registration isn't an option on the desktop client, even if I also have a phone number. I looked for a fork or alternate compatible client that allows registration but didn't find one.
Signal's privacy policy says "Signal is designed to never collect or store any sensitive information." but that hasn't been true for a long time now. They just refuse to update their policy; maybe that's them trying to warn people away from the service as loudly as they can.
https://web.archive.org/web/20230519120156/https://community...
I notice the README file for signal-cli [1] says registration "may require solving a CAPTCHA challenge", which might involve running proprietary software. I found a forum post that says this also sometimes happens on sign-in [2], which might make it difficult to use Signal without proprietary software. I might try it anyway at some point because it does say "may" :)
[1] https://github.com/AsamK/signal-cli [2] https://community.e.foundation/t/signal-and-hcaptcha/83470