> Sybil identities is currently an unsolved problem, the mitigation is the requirement of unique scarce resources (like phone number in this case)
Then let your phone number receive the spam instead?
Then let your phone number receive the spam instead?
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.