I used to work on a related problem (call traffic pumping) for a large VOIP provider, for domestic & international calls. It was a very interesting problem whic

I used to work on a related problem (call traffic pumping) for a large VOIP provider, for domestic & international calls. It was a very interesting problem which has two distinct patterns of abuse. First is just basic account takeover/hijacking, where a criminal will login to your account and receive a cut for calling expensive numbers (or in this case, sending SMS's to them). This is basically the web version of the pre-existing PBX hijacking issue. The second, more interesting version, is people abusing free quotas to send calls (or SMS), such as in the verification case. This is novel (relative to the pre-web era) because historically it would have been hard to make free calls/texts.