How to fake Roaming?

Hi We are planing to combine OpenBSC with our wifi mesh implementation in a static setup as a demo / testbed. We would like to implement a roaming like behavior and have actually two approaches 1. Configuring a call forward on the phone so that it the "real" provider forwards the call to a number that we can route into the openBSC based network if the phone is not connected to its network. This would allow people to call "me" eager if i connected to the "real" network or the OpenBSC network. Disadvantage would be that if I call somebody from the OpenBSC network the called party would not see the number from the "real" network. 2. Use a service like sipgate one. It would also allow a unified number for both "real" and OpenBSC network but if you call from the "real" network it has a different number. Besides the unlikely way of real roaming with an carrier another option would be to fake the outgoing number. Does anybody know a service that could be used for that (i would need to fake a german GSM number). Does anybody know how this is handled on oil rigs or ships? Do they have a roaming agreement ? thanks mfg Peter