If you remove the call forward from your voip provider and set your OBi to something like this:
Voice Services > SP2 Service > X_InboundCallRoute:
{sp1(12345678901)}
Where 12345678901 is your cell phone number.
Any call coming in on sp2 would be routed via sp1 to your cell phone. Most voip providers allow two simultaneous calls so this would probably work too:
Voice Services > SP2 Service > X_InboundCallRoute:
{sp2(12345678901)}
In both case no Obi connected phones will ring as there is no "ph" in the InboundCallRoute.
Of course, the above scenarios cost you an outgoing call from your OBi to your service provider.
If you stay with your present setup, then there are some bizarre ways of sending ringing into nowhere by setting up a fake Voice Gateway and sending the call there. If you wish to go to those lengths I'm happy to post a solution if you request it. That's for tomorrow though, it's getting late in my time zone