I am trying to DIY why a phone on my TDA50 stopped working after a technician did an alarm service call. 3 phone (non Panasonic) tech have been out and cannot find the problem.

The phone in my study stopped working. There is a red line on every other phone like that phone is in use.

I have done the following:

Swapped the phone between another station the phone works at the other station and when pugged into the PBX. I have tried plugging in another phone in the study with no success. While the phone is using pins 3&4 on the -11 connector, this line uses pins 2&5 when connected to the PBX.

I have disconnected the line from the phone and the PBX and checked the continuity and the line does not show a short. I have pinged the line from the phone jack through the terminal blocks and I get a strong tone when it is disconnected from the PBX. I have disconnected the line from the PBX and the phone. Tested the line with a 9 volts battery and I get a strong 9 volts on the meter.

I swapped the connector at the PBX with another location and the other phone (Bedroom) shows a red light on all of the phones, but the phone in the bedroom works. But the phone in the study does not work. So the terminal on the card is fine.

SO;

The phone works
The line seems to be ok from the phone, through the terminal blocks to the PBX.
The card seems to be fine.

What am I missing?

Thanks in advance for the help.

R