!1900xxx xxxx – Barring all 11-digit numbers that start with 1900 (straight from the OBi guide)
If this doesn't work, there may be a problem.
If the call still goes throught remember that the digi map reads from left to right. If your dial plan is complex you may not have caught it yet. You didn't give any details, no need to tell you to look in the following Outbound Settings to check where the rule you would like to implement is located.
Here is a post from our resident digimap expert. We like to keep his memory alive since he has been on hiatus for awhile.
Re: Dial plan explanation http://www.obitalk.com/forum/index.php?topic=34.msg10735#msg10735