Isn't that always the case?

They make it sound like it just happened.
If all other remote notification users are working properly, then it would have to be something with their programming. Maybe they forgot to put a 9 in front of the pager number, or maybe they have a pin number required. They could have also selected the wrong type of pager in the remote notification settings.
My suggestion would be to look at their programming by going in to Meridian Mail, go to User Administration, View/Modify, enter in DN, page down and line up cursor to Remote Notification Schedules, Press F8 for more details, verify current state is on, verify the times are enabled, check the target DN's to see if they have the correct pager number, check which of Phone, Tone, Voice, Numeric, or Service is selected.
Check these first and let us know. Thanks.
Mike