Assuming you have an additional number from the carrier already. Use the same pattern length as the numbers that are already there (typically 4 digits). On other thing: you may find a "+", meaning any number not in the list, and an "E", meaning no DID number at all, at the bottom of the list. If you see them, then since the numbers hunt top down if you add a new number you'll have to rebuild the the bottom of the list so these remain last.