Does your provider allow you to send an originating CID other than the one that is assigned to you. I know that many don't. In some countries it is actually forbidden (by law) to spoof caller ID, which is, in reality, what you are attempting to do.
Choose an extension, in the settings you can put a number to be displayed. Normally this field is blank. I found that if I put any number in there, that is what is displayed to the called party on an outside call. My provider simply passes it along. If you change that, does the call still go through, or is it blocked? if it is blocked than you need to ask your provider if they are blocking calls with an "incorrect" CID. If that is the case, then you are out of luck.
