Need some more information but i am assuming you are just trying to get the same caller ID for all calls outbound.
Here are some options (these are brief and may or may not be suitable for your environment):
- Add the same external mask to all phone lines (this will be what is shown at the top line of the phone)
- Change the calling party number on the route patterns used for calls going out
- Do a calling party transformation mask that is applied to the trunk
- Do a translation on the gateway itself with a voice translation rule/profile
The number you put in the directory does not affect caller ID, it just what users use to reach another user as long as there is a corresponding DN or pattern to reach them