What to check

>When the release button is pressed on a phone it sends a series of DTMF tones on the speech lines.

The door unit "hears" these tones and then sends a serial data code to the lock relay in the power supply on wire D6.

Up to 2021 - The visitor should hear only hear the first DTMF burst, then the system should Mute the speaker & microphone, and then it checks for the second burst to verify the tones are coming from a phone.

From 2021 (ver 4.12) - The visitor should only hear a very short beginning of the tones.

Then, having verified the tones, it sends a serial data code to the lock relay in the power supply on wire D6.

However, if the volume is too loud and the tones get distorted, it doesn't recognise the first burst, and so does not mute, the user then hears the second burst, and as it was not verified, no lock command is sent to the lock relay.

Corrective Action

Reduce volume of SPK and or MIC