I haven't actually tried this, but you should be able to do some fairly fancy things with the OBiTALK Service InboundCallRoute. For example:
{(200123456|200654321)>(1xxxxxxxxxx):li},{(200123456|200654321)>(011xx.):sp2},{aa}
This should allow OBiTALK callers 200123456 and 200654321 to make direct calls, with US/Canada calls being directed to the LINE Port and international calls being directed to the SP2 Service. All others would be sent to the Auto Attendant, where they could make 2-stage calls, including calls to your OBi telephone by dialing 0 (use PIN protection to ensure only authorized users can make calls through the Auto Attendant).