Comment by mrbiber

I'm surprised to see that matrix.org / riot.im is not mentioned as an alternative to Signal. They have a federated, open protocol, usable open-source apps for web/Android/iOS, registration without a phone number (even email is optional), do not require Google Play, and support double-ratchet encryption (in beta, it's not activated by default yet, but it will be activated by default in private rooms in the future).