Put the number in External Phone Number Mask on the Directory Number page near the bottom. The non your Route Patterns click on the box that says Use Phones External Number Mask. As long as it's not set on the Gateway or at the provider, it should work.
Be aware that if you put it in the External Phone Number Mask field, that number will show up on the black bar at the top of the multi button phones display screen. If you use a 7905/7911, single button phone it will over write the directory number on the phone. Just put the DN in the Ascii Text Label box and it will come back.