How to Troubleshoot "Phone Line Fail" D7112 D128

When does the D128 dual phone line switcher module indicate a phone line failure?

 

 

An amber phone line status LED lights on the D128 module when the control/communicator measures less than 25VDC or 10mA of current across the tip and ring of the telephone line. The D8112 and D9112 monitor the telephone lines for a programmable interval before indicating a trouble condition.