Dial on demand problem

I have a very small client that is using a shared modem on a Win2K server. They have very limited access needs and only use it for checking email several times a day. We have it set to dial on demand. The server seams to have a mind of it’s own and will dial at what appears to be random times, even in the middle of the night when no one is in the office. How do I get it to only dial when IE is loaded or they are checking email??? Then hang up when it is no longer needed?