Comment by bpfrh

No, the phone number needs to be known by the other party and you need to accept the "friend" request.

It prevents the creation of an unlimited number of signal accounts by a single user with no cost to the user but cost to signal and other signal users.

edit: Your are probably right in that it does not change the risk of spam for a single user, as you could guess the phone number or just iterate over all known phone numbers and try to connect to them.

requiring phone numbers only solves the cost problem for signal(The company/legal entity) and lowers(hopefully) the amount of spam that would get send.