I wasn't aware that Google Voice was selling cell phone service, but that's not important to this problem.
Assuming you have a regular Google Voice account configured on an OBi as SP1...
When you get an incoming call on SP1, it will naturally ring the telephone connected to the OBi's PHONE Port.
If you want to simultaneously ring one or more other phones, be they landlines, cell phones, regular Google Voice accounts, VoIP numbers, or whatever, the OBi does this by making one or more totally separate outgoing calls.
So, for an incoming call to the Google Voice account configured on SP1 to simultaneously ring two cell phones in addition to the OBi PHONE Port, the OBi has to initiate two outgoing calls to those cell phones. Whichever called destination answers first is bridged to the incoming SP1 call and the others are abandoned.
Each Google Voice account can accomodate two sessions, so if you only need to ring one cell phone in addition to the PHONE Port when receiving an incoming call, you could get by with only the Google Voice account on SP1:
Voice Services -> SP1 Service -> X_InboundCallRoute : {ph,sp1(12345559991)}
If you need to ring two cell phones in addition to the PHONE Port when receiving an incoming call, you will need additional resources.