I'm assuming that you have DID numbers, and you want the DID, with the new prefix to go out for each call.
Have you confirmed that your provider allows you to change the caller ID to something other than the number(s) the you are paying for? This includes adding digits. Some don't allow any changes.
In each extension setting, under Other, there is a field called Outbound caller ID. If the outbound settings in the trunk are correct for your provider (you may have to change these), and they allow it, that number should be displayed.
There may be a way, in version 15, to add a prefix to all numbers, but I'm not aware of it.