T28P Handset Noise Problems

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I came across this issue with one handset last week.
I found that when the curly cord is closer to other electrical wires, the hum/buzzing gets worse. I swapped the curly cord with a known good one, and the problem persisted, indicating the fault lies in the microphone.

As others have done, unplugging the handset for a period of time resolves the problem.

It seems that the electret condenser microphone in the Yealink handset can develop an over sensitivity to hum fields if left powered on for extended periods. The microphone is always powered - I discovered this by putting the headset up to my ear whilst holding down the hook switch with my other hand, I can still hear sidetone.

Maybe if an electrical bias develops on the microphone, causing an imbalance on the two microphone wires, makes it is more susceptible to picking up hum and interference. My theory is that disconnecting the handset allows the charge to dissipate in the dielectric of the condenser microphone, allowing it work normally again.

Maybe a potential fix (that the Yealink engineers can apply to new firmware) is to turn off the power supply to the microphone when the handset is on hook?