Lots of companies over the years have thrived on undercutting incumbent operators’ prices for long-distance and international calls. Calling cards and callback systems are the two oldest and most common examples; VoIP is a more recent one. Lots of companies have targeted the mobile market as well, either just through marketing, or with products geared towards mobile users, but their underlying proposition remains the same: cheaper calls.
Last summer, which (among other things) gave users local numbers for their international contacts. So instead of dialing the international number, you’d dial the local number, then the call would get routed through Rebtel’s VoIP network at cheaper rates. It’s a novel idea, and apparently one worth copying, as another mobile VoIP company, Jajah, has announced a similar feature.