If someone is setting up a SIP account, Obi wizard can ask for local area code and rewrite the digimap for direct dialing instead of dialing the Obi prefixes (in the Service Providers=>ITSP Profile A=>General=>DigitMap) ex: 514xxxxxxx. I think most users would prefer this behavior. Obi already has ability to rewrite digimap.
If you know where to look this can still be accomplished. However I think you are aiming the Obi product to general users who might not know how to configure this behavior.
To clarify, I think you are looking for the OBi wizard to help you craft digitmaps and outbound call routing rules such that the user by associating area codes, country codes, etc. can define the trunk on which the call will route.