This practice of creating a lot of traffic on rural lines is known as traffic pumping. AFAIK the FCC has closed this loophole, stating that termination fees may not be collected for VOIP calls. IIRC this new rule will take effect in 2014.

BTW, FreeConferenceCalls know their model is doomed and are expanding as fast as they can into countries with phone systems modeled after the US one, where the loophole still persists. It's pretty clear, though, that their business isn't sustainable. My guess is that somebody will soon gobble up FreeConferenceCall for their customer database.