I pulled up the TDA50/G Installation manual to first check the allowable distance for wiring.

The smallest figure quoted for using "other" PTs or a DSS Console is 458ft using 26AWG wire, so you are definitely good there.

Is the conduit steel, or PVC? Is it possible water could have entered the conduit and caused damage? Is there anything else run in the conduit besides telephone pairs? Hopefully there is nothing over 24 volts.

You might consider ringing out the pairs in the conduit to check for oddly high resistance as well as intermittently shorting pair(s).

Check the static/lightning protection unit closest to the PBX. Repeated static, spikes, and electrical activity hits will weaken the MOVs in surge protection units over time and eventually break them down.

Give it the "smell" test for burned parts as well as physical inspection.

If all this checks out to be okay, I think you may have to call Panasonic support. Have your Dealer ID # handy.

Unfortunately in a few more months the world of Panasonic PBX systems will end. I have no idea how dealers will continue to support existing product after the final cut off date.