What to check

Each video distributor output may be feeding one phone or a chain of phones.

If the terminating link(s) in the phones are set correctly, the output pair 1&2 should have a resistance of about 100 Ohms which is the terminating link at the end of the chain.

A short between 5(24v+) and 1 or 2 will damage the distributor output and the video receiver in the phone.

Corrective Action

Use a spare output if available, or replace the video distributor PCB.

500-VS4 or DA500R

Links

https://www.entryphone.co.uk/shop/productdetail/PCB-500-VS4

https://www.entryphone.co.uk/shop/productdetail/PCB-DA500-R

https://www.entryphone.co.uk/shop/productdetail/PCB-500PSU-R