I would love it if you could make a windows phone 7 app. Shouldn't be that hard and most likely will port to windows 8!!!! Please.
For use under Windows (including 7), Obihai provides OBiAPP for PC (http://www.obitalk.com/forum/index.php?board=9.0) to interface a SIP client such as X-Lite to the OBiTALK network.
A SIP client such as X-Lite can also be used without OBiAPP for PC (http://www.obitalk.com/forum/index.php?board=9.0) using this configuration:
Single-Stage Dialing Through Any OBi Trunk Using SIP (http://www.obitalk.com/forum/index.php?topic=2454.0)