Comment by shp0ngle
2 years ago
They seem to actually confirm it themselves, which I didn't know.
> We use third-party services to send a registration code via SMS or voice call in order to verify that the person in possession of a given phone number actually intended to sign up for a Signal account. This is a critical step in helping to prevent spam accounts from signing up for the service and rendering it completely unusable—a non-trivial problem for any popular messaging app.
From their blogpost
I'm sure they absolutely do use it as a spam signal. You use what you've got. But that's not why they use phone number identifiers, which is a design decision they and others have written ad nauseam about.