Spam prevention.
Discussion
Seems like a terrible spam prevention measure for a privacy service. I'm sure it's effective at prevwnting spam but also effective at KYCing users. Nothing could be worse for privacy.
You may not know this but you can acquire a phone number without KYC. And its actually been very good at stopping spam. Better than any other messaging system I've used. It is all about trade offs.
Of course you can, but most people don't. Which means Signal has metadata on 99% of users. They know who's talking to who.