Is the phone line going directly from the back of the alarm dialer to the telephone wall jack?
If you have a phone splitter, remove that and see if the call goes through. If you have DSL phone service, make sure your lines and/or the phone jack you are using has a DSL filter on it. If all else fails, there may be a problem with the alarm dialer and/or your phone service. Contact Customer Service for more information on additional testing options.