It is early stage, but there is now an alternative to Signal that doesn't use phone numbers at all: https://xx.network/messenger/ While you can add your number to be searchable by others, it doesn't let strangers with your number know you signed up automatically, either. Full disclosure: I work on the infrastructure behind it.
> Full disclosure: I work on the infrastructure behind it.
Oh cool... I ran a node for many months during the beta (from home, fiber optic at home). I'm busy atm so I'm not running anything anymore but I do really hope that a real secure messenger that doesn't leak metadata left and right, and which uses advanced cryptography, shall eventually prevail.