Hi,
The firmware is provided to 3CX directly from the manufacturer, you will have to contact their support center directly about it since it is not something we provision or have control over.
For now you have a couple of options:
1. When you enter the DTMF code, the device plays a melody to inform the visitor that the door is open. If you hang up before the melody stops playing, it will continue to play in a loop. So wait for it to stop before hanging up and it should be ok.
2. Better yet: you can disable the melody entirely from here and it will stay that way
View attachment 49592
We will also inform Fanvil about this, but since it is not a 3CX-related issue in particular, we highly recommend that you open a case with them too, as a Fanvil customer so they can have direct feedback and be aware of the issues that affect their users.