Cordless phone buzzing noise

Our old BT Freestyle three-handset plus answering machine is dying, with its displays failing so only part of each number is visible. We bought a new BT7610 Trio to replace it, but we hear a buzzing sound when we use it � our callers don't.

I tried BT Quiet Line and the buzzing (mains frequency I think) remains audible until I move within 1 metre of the base station. I spent the morning unplugging everything (computers, Devolo homeplugs, router etc) and plugging the phone into the test socket � no difference. The buzz fades in and out at about 10-second intervals but it's not acceptable for daily use even though we like the 7610 otherwise.

I am returning the phone as faulty, because our old unit still works perfectly when replaced. From the net, it seems that background buzz is not uncommon. Has anyone any ideas, please?

Agree it sounds like a faulty phone.