The Signal messenger and protocol.

@Dienervent @andrewm @citytree @animist @BananaTrifleViolin
Signal was originally designed with phone number as the key identifier. This is to facilitate contact identification and building the social graph. The real issue is not that you need a phone number to register with signal. The problem is signal exposes the phone number to all people with whom you communicate, including in groups. That is a big privacy concern, especially if you a part of large groups where you don't want everyone to see your phone number. This is a well known issue and the solution is to have disposable usernames along with ability to hide phone numbers from contacts.
Signal is currently working on these and some previews are available already. Hopefully that should be released soon.