I used an adapter to convert the Nokia 4 conductor headset to a standard 3 conductor headset with noise cancelling mike and get a feedback once the call goes through. With a 4 conductor earbud there is no feedback. This problem exists with different adapters and headsets. Is there any way to adjust microphone sensitivity or eliminate the problem in another way. I could not find any reference in the "setup" menu or documentation for different settings for the headset. Any ideas would be greatly appreciated as I have a bunch of these phones to maintain.

P.S. The phone is on the T-Mobile network with their software in the phone/SIM card.