D305 mains humming noise

Dear Members, I have two sites where the SNOM D305 used with VOIP PCBs of our make (Designa) and any call emits at both end a loud 50 Hz hum, making phonecalls unhearable. One site clearly has a grounding problem, the hum with some interference can be heard in the local broadcast system too.

The hum is not present when I open the line by picking up the handset or pressing speaker. It starts when I call a client and the line established between them.

Of course I have tried everything: phone PSU replacement, phone replacement to another D305, UTP cables with cut POE lanes, running the phone off from an UPS.

One strange thing I noticed when I pull out UTP cable during a call, the hum remains there-maybe not a symptom, but a result.

And the weirdest note: I replaced the phone to a D300 and the hum is gone. I also made a test with the subjected D305 at another site-the phone is working without any hum.

So for me it looks like the D305 has some more sensitivity to mains network problem while the D300 is still able to work better in such environment.

I need any advice as already two customers have such problem and I have to find a solution over than have alook on mains network.