But putting a phone number in immediately exposes protesters to association. Sure, Signal can’t give out the contents of messages, but it still has the chain of contact. So if a government gets hold of this record, legally or otherwise, now you have everyone associated to a suspect phone number/person and can start rounding them up.
It’s the complete antithesis of freedom of association when there’s a record of everyone that you’ve contacted. The contents don’t enter into that problem, and I can’t see why they feel the need to keep this as part of their system. It purposely makes it impossible to use this for something like peaceful protest. So, no, it doesn’t give you privacy from governments, because governments that don’t respect freedom of association will use that information to punish dissidents.
I can’t imagine any reason to use phone numbers except to purposefully keep this chain of association for governments to use. Even Facebook doesn’t require this sort of personal proof, and it’s suspicious as hell.