If you are experiencing any problem with a telephone you need to determine if the problem is in the phone or in an external source such as the control unit or the wiring.

Try swapping just the main part of the phone (leaving the handset and all cords behind) with another phone, preferably of the same exact model. If the problem follows the phone and the good working phone still works in its new location, then you just eliminated most possible external sources.

Troubleshooting 101