[throwawaycuriou]

Unrelated to this current focus on the weakness of using a phone number as a contact reference, I have experienced a different problem. If a user has ever used Signal previously and for whatever reason revert back to SMS, if another user sends them a message via Signal it is lost with no indication. This is a growing problem as people try out Signal and when they get a new phone just forget or don't care to install it again.