Google requires a phone number for most users to create a new account; there are currently ways around this, but for obvious reasons, I have no intention of sharing how to do this.

Google Voice has always required a phone number for signup, since its legacy design was to forward calls. When creating the account you’re able to allow Google to generate a Google Voice number for the account, disconnect the setup number, and use Google without a phone number.

Why though go to all that trouble when Signal just needs a PIN one-time from a throw away number; at which point Signal allows user to lockout future registrations from that number as long as the Signal account is active per their definition.