Usually I will add the main number in the default number. If the PBX doesn't send up a phone number then the carrier will usually attach the BTN (main number). If you assign the last 7 digits of the caller ID to the extension, then the system will add the default area code along and send the full 10 digits to the carrier.

It's a major pain when you have some extensions that want to send out a different area code. Toshiba did this a little better in the IPEdge.