It could be a number of things.
Either there is aproblem with the actual unit itself. Maybe the battery.
Or the telephone socket which you are plugging it into is not working correctly.
If you open the socket by unscrewing the 2 fixing screws, you should see a small component board in the socket.
On this board there will be a barrelled shape component called a capacitor. This barrell shape component is the part of the socket which generates ring voltage.
You may need to try another socket.
patrick_roelake@yahoo.com, July 2006