When you use an OBiTalk wizard to setup a trunk and you supply an area code for 7 digit dialing, OBiTalk will add a rule to the configuration to prepend the area code to a 7 digit number.
Assuming GV is defined on SP1.
Enter OBi Expert.
Go to:
Service Providers -> ITSP Profile A General -> DigitMap
Uncheck both boxes to the right of the Value.
Now look at the DigitMap Value.
You should find a rule like: <1870>xxxxxxx
This rule will prepend "1870" to a 7 digit number.
There's no need to go into OBi expert mode. The (optional) setting is right on the basic portal page for configuring a service provider (SP) -- see the attached screenshot.
The OP enabled this feature during setup. So, as you point out, GV is adding the area code if only 7 digits are "dialed" by the user. I should have explained that is the reason that GV seemingly doesn't care about the area code -- it does, and the OBi digit map is now adding it when dialing 7 digits, and not adding it when the caller dials 10 or 11 digits. My point was, in this user's case, it doesn't matter whether or not the call is "long distance" from the old POTS industry viewpoint, all calls need to be sent to the service provider with 10 or 11 digits.