Would that mean that they could break the forced use of a phone number to register with them? That was something I found incredibly offensive about signal (and one of the reasons I would NEVER use it), which is that your real phone number has to be registered with them in order to deliver messages. End to end encrypted or not, if their server registers a message sequence passing between your phone number and one registered to a known child molester (hopefully you don't know any, but maybe somebody you communicate with for business and don't realize?) then you don't know who has access to that server. Their "word" isn't worth anything.