If you don't have Outgoing Line Identifier (OLI) programmed for a particular set then the telco will send the default caller ID number, most commonly the first DID number you have. You should have your telco set your reception number as that default.
You can certainly set individual OLIs, but I wouldn't depend on that as the default.
In my case, the DID block runs from 1180 to 1279 (don't ask me why...), so I had to (very nicely) ask our telco to send 1200 as the default caller ID number, which they did. The trick is talking to someone who understands what you want, and how to order it--after that it's easy.
emk