You had mentioned that your cell plan includes unlimited calling to your office number. Is more than one such number allowed? If so, you could set up Localphone on your OBi and dial a local DO number on your mobile. Since this would be coming in as SIP, there would be no delay for caller ID. You probably would not hear any ringing until Vonage picked up. Used this way, there would be no charge from Localphone.
Alternatively, you could set Vonage Extension as a contact in Localphone; calling the DO number would ring directly to Vonage, bypassing both your OBi and GV. This would likely result in better call quality, lower latency and yet faster setup time. However, it's not free, as Localphone charges $0.005/min. to call a US number.