Are you sure the caller isn't blocking their ID? Lots of places set their sysems up so their CLID doesn't go out and a lot of cell phones don't send their CLID either. Then there are the people who block it on a call-by-call basis using *XX from home.
Fact of the matter is, if it gets sent to you, you will display it. It's important to look at the monitor when the call is up. If the info is there and it;s not being displayed, that's another problem.