How to Diagnose "Phone Line Fail" D7212B & D9112B Panels

What voltage and current is the phone line monitor measuring to sense trouble on the phone line on the D7212B/D9112B panels?

 

 

The phone line monitor senses trouble when the voltage on the line falls below 4.5 to 7.5 VDC, or in the off-hook condition when the current is 8 to 13mA.