I would be surprised if this doesn't constitute a violation of Terms of Service somewhere along the line? Something that could provide grounds for one of the telecom companies involved to have it taken down, if it were to be successful.

Basically, it means that you can send information (in one direction only), without paying for use of the telecom network. Taken to an extreme, it should be possible to create a "free walkie talkie" app that worked all over the world, for free, by repeatedly calling back and forth but never connecting the call.