CP860 change sensitivity of Microphones

Hi,

Is it possible to adjust the sensitivity of the microphones on the CP860.
Users report that remote parties often complain of quiet transmission and ask the users to repeat louder.

We are testing with CP860-37.80.0.10 and have tried earlier releases too.

I have tried - and failed to adjust using the voice.handfree_send provisioning parameter.

thanks

Adam