Today's Wall Street Journal has a story highlighting client Truphone and a few others in the alternative mobile Long Distance biz.
Jajah, Mobivox, Gizmo project and a few others are mentioned in the well researched and thorough must read on the subject of Mobile VoIP.
From my position Truphone is really much akin to being an alternative long distance provider, but unlike MCI and Sprint of days gone by they have figured out how they can be both carrier cooperative in that they do drive minutes to the mobile operator, as well as user friendly in how they approach long distance and International calling via VOIP and when available, a WiFi connection.
While they remain WiFi calling first, and the best at it, their iPhone clients uses the iPhone's real phone number, while calls back to it go over the cellular network. Their Truphone Anywhere plan means that calls when not in range of a hotspot use minutes from the in place calling plan.