What to check

The phones should sends at least two bursts of DTMF tones to the door, to open the door, the lock button on the phone should have some 'tactile click' feel to it when pressed as well as hearing tones in the handset/speaker.

Corrective Action

Replace the phone unit PCB.

Additional notes

PCC-501/531xx

Links

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

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

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