for clarity, you don't have google voice set up on sp1. You have your google voice calls forwarded to CC which is set up on sp1.
There should be no problem forwarding the calls to your google voice number with either CC or
callwithus.com, as long as they support outgoing calls. And also to forward using the same service the call comes in on, your service provider would have to support at least two simultaneous sessions.
Also, in order to forward busy calls at the Obi; your service providers will need to support at least two simultaneous sessions and you will need to disable call waiting for the phone port.