Hi
By default with the IP Office, if a user has a DDI number, that is usually the number that is presented when dialling out. If the user has no DDI, they present the main billing number. (I am assuming that your PRI will operate the same as in the UK)
If you need to manually set the OG presentation, for each user, create a shortcode in the User _> Shortcode tab such as
SC ?
Number .s<number to be presented>
Fearure Dial.
Sometimes, when crosiing telco's, the modified number can get stripped out, and just the main billing number gets presented.
You will also need to check with your telco to see if they support the end user modifying the number to present.
Hope that helps!
Dave