It's illegal in the vast majority of countries [1] including Germany, France. Attempting to build a safe secure communication system around phone numbers that is suitable for situations like this, relying on them as being somehow anonymous, is just a complete non-starter. Even if you don't have to register the phone number, the cell provider will still know your location history. I think it just shows how America-centric Signal is.

[1] https://privacyinternational.org/long-read/3018/timeline-sim...