On the user in question, go to short codes. Add a code with the following:
Code: ?D
Telephone Number= the extension or number to be dialed
Feature (if internal ext Feature should be Dial Extn. If External number Feature should be Dial (Line Group ID will be the line group of your ARS table being used, EG 51)
Then your ARS table 51 needs to be configured to SEND the appropriate outbound caller ID.
Code: N;
Telephone number: NS5555555555
Feature: Dial3k1
Line Group ID: Line group of PRI outgoing traffic
If this is SIP, then the system can be configured to use the SIP tab of the user for the outbound caller ID information.