My customer has a UC520 and CP-524G IP phones. They are currently using Plantronics CS50 headsets. On the phones that have the headset connected, if they ever pick up the handset to answer a call (instead of using the headset) they get a fairly loud, repetitive humming/vibrating noise that can be heard on both ends of the call. This doesn't happen if they leave the handset down and pickup the call with the headset. It only happens if they have a headset connected, but pick up the handset to answer a call. If they completely unplug the Plantronics headset, there is no problem. I have to believe there's a compatible headset or at least some way to make things work with the Plantronics CS50 headset where they can use either the headset or the handset and not have any humming noises.
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