The Signal messenger and protocol.

Phone numbers keep spam down. It’s much much more difficult getting a bunch of phone numbers than it is to get emails so you mass create accounts

I don't see that as a problem. They basically only save the phone number, creation date and date of the last connection to the server.

Signal was only able to provide two sets of data: the timestamp of when the account was created and the date of the last connection to the Signal server, as Signal does not store any other information about its users. source