Have you read the OBi Device Admin Guide available at
http://www.obihai.com/docs-downloads ?
There are many options. The simplest is to have calls coming in on each SP ring the OBi's phone and your cellphone simultaneously. You can answer on either.
Under the respective Voice Service find the X_InboundCallRoute. The default is ph. Change that to {ph,spx(1234567890)} where the x is the SP you want to use for the call to the cellphone and 1234567890 is the cellphone number. Note the following:
1. You don't really need to do this for the GV slot as GV is itself capable of ringing 6 destinations in parallel.
2. Your cellphone will not display the number of your caller. It will show your phone number of the SP you are using.
3. This will not stop incoming GV calls going to GV voicemail if unanswered within the usual about 25 seconds approximately.