Question about Poly Voyager Legend + Case

I bought 2 Voyager Legends for the shop. Both developed a static buzz after a short time. Torture for the ears! Funnily enough, even when the power switch is set to off. In addition, one of the headsets constantly makes itself heard. When a call comes in, it immediately answers the call, even if the headset is lying on the table, for example. I can't find any troubleshooting tips. Does anyone know these problems? I'll throw them in the bin now.