You don't want to have to use a payment method at all... the problem with not having a phone number is there is no way to tie the account to someone who is responsible for the content, and Signal does not want the liability nor the spam traffic, of potentially harmful traffic (in whatever sense that may mean).
Or if you want the tinfoil hat response... they need/want to tie your account to a real identity for surveillance purposes, and they can't do that if there is no phone number, so they use a payment record instead. (I made all that up and have no idea if it's true)