Thank you for mention this! I don't know why Wire is not mentioned in thread like this. It is best without meta data collection (such as phone number). You can register with just an email and it is based on the encryption protocol that Signal uses. On top of that, the server is written in Haskell!!! Yes, Signal server is in Java, btw. Which is not bad. And Wire is based in Switzerland, with GDPR in Europe it has better data privacy jurisdictions.