Hello! I'm having exactly the same problem, but in reverse! My SIP phone rings fine when my GV number is called, but I can't seem to call out. I used SP4 to serve as my SIP proxy to SP1 for GV.
1001 is my SIP ID. So my X_InboundCallRoute looks like {1001>(Msp1):SP1}
and when I dial a number from my SIP phone, I get nothing.
When my GV number rings, my SIP phone rings and I can answer and talk, but I just can't call out.
My SP1, GV X_InboundCallRoute is: {ph,sp4(1001)}