A bit handwavy, but allowing sign-up without a phone number could massively increase bot/spam traffic and ultimately increase hosting costs for Signal.
Accepting these payments would not be trivial, and linking them to Signal accounts would create a treasure trove of metadata that neither Signal nor its users would likely be very happy about.
Just charge $10 to create an account without a phone number and accept Bitcoin. Most people can avoid the $10 by providing a phone number, privacy-conscious people only have to pay $10, it generates revenue, and the $10 puts the spammers out of business because they don't pay $10 once, they pay $10 every time they get banned, which happens multiple times a day.
You could even automate the bans by banning anyone who gets blocked by more than two people they sent messages to, which anybody can avoid by not sending messages to people who would block them, and if it happens to someone innocent, it's still only another $10 to reactivate your account.
