Unfortunately, spam exists and phone verification is one of the least-bad-way to ensure that the user is a real person (there are other options, but it really is one that has many advantages).
Given that Signal does not have access (by design) to much information about their users when they use the service, they can't really fight spam once accounts are created. You could do spam detection on the client and privacy-preserving voting in order to ban spammers, but the UX would be very poor and that opens a whole new can of worms.
Given that Signal does not have access (by design) to much information about their users when they use the service, they can't really fight spam once accounts are created. You could do spam detection on the client and privacy-preserving voting in order to ban spammers, but the UX would be very poor and that opens a whole new can of worms.