GV will accept 10 or 11 digit numbers. If you have another provider on SP2, you can also use them. I would use the OBiTalk to call another OBi device. pp designates the OBiTalk network. The *0 will send a zero as data to the other OBi. Replace 510123456 with the OBi number of the receiving OBi.
{0:sp1(8005551212),sp1(8005551313),sp2(8005551414),pp(510123456*0)}
In the receiving OBi you can check for the zero and route the call to the attached phone and 3 more devices.
Voice Services -> OBiTalk Service -> InboundCallRoute:
{510654321>0:ph,sp1(8005551515),sp1(8005551616),sp1(8005551717)}
Replace 510654321 with the OBi number of the sending OBi
Again, when someone answers one of the calls, all the other calls will be terminated.