AFAIK you have to maintain the phone number you sign up with to be able to keep your "account". In many places, it's not feasible to keep a second phone number just for signal simply because it's too expensive.

You need to provide government issued ID in many places to buy a SIM. For example, I had to scan my passport to get a SIM card in Madrid from a vending machine a few months ago

Only if all of your contacts, and their contacts, and so on, also do the same.