Only problem: if a user (A) signs up for signal and then uninstalls, the phone number is still tied to the signal app. This means that a signal user (B) now can't get messages to user A. The messages gets delivered to a dead signal account. User A must manually deactivate her account, which she is not even informed about when deleting. Also: if user A has her number trabsferred too somebody else (which is often the case with e g work phones) her account is now owned by a random person.