Truphone, Gizmo Project, Wifimobile and the like should simply make the Sipbroker Callthrough numbers a part of their mobile applications to make them usable outside of Wifi.
I guess that's already it.
People would always only pay a local call and the international part goes over the respective VoIP provider. The mobile application should always "sense" in which country the cell phone is situated and choose the local Sipbroker number to establish the call.