This is a complex issue, depending on your router and the capabilities of the external source.
In general, if you forward the UDP port specified by the X_UserAgentPort setting for the SPx in question to the private IP address of the OBi, an external call will look like an incoming call to SPx (though with a different destination number) and you can set the InboundCallRoute to route it as desired.
For more specific advice, post details about your application, e.g. "I'd like calls to my IPKall DID to ring my cell phone via GV."