we have a D375, firmware 10.1.57.14, where the microphone of the handset is not working at all.
I can hear the caller, however, the caller can't hear me. Further, I hear myself in the handset.
We have updated the firmware from v8.x.x to the current one. We have exchanged the cable from the handset, we have exchanged the handset itself, we have resetted to default settings, everything with no effect.
Here the settings:
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For this issue I don't think there is much to do, if even with another phone's handset and cable this is not working I think you should contact your reseller for an RMA of the phone.