If you set unconditional call forwarding from the phone port all calls, including the PSTN calls, will be forwarded.
I think the only way to forward on the calls for SP1 would be to set it in the SP1 calling features. This can be from the Obi Expert Configuration or in the device itself.
Voice Service -> SP1 Service -> Calling Features -> CallForwardUnconditionalNumber = pp(300xxxxxx)
that can remain all the time. Then when you want to forward you enable forwarding
Voice Service -> SP1 Service -> Calling Features -> CallForwardUnconditionalEnable
Perhaps you could just fork the calls on SP! to the other obi all the time, and answer them in whichever location you are at.
Voice Service -> SP1 Service -> X_InboundCallRoute = {ph,pp(300xxxxxx)}
you could just turn the ringer off on the phone when you leave.