Because you need something paid for underneath it to stop spam. This fantastic post from Mike Hearn, who was on the GMail spam team and now makes P2P decentarl

Because you need something paid for underneath it to stop spam. This fantastic post from Mike Hearn, who was on the GMail spam team and now makes P2P decentarlized stuff, explains it all: https://moderncrypto.org/mail-archive/messaging/2014/000780.html https://moderncrypto.org/mail-archive/messaging/2014/000780.... Essentially, every large provider (e.g. Facebook, GMail) of network services ultimately uses phone numbers now as a common thing that can't be infinitely reproduced. They underpin all high quality antispam on large Internet systems.