This not a configurable value. When you lift the handset offhook, the audio and microphone are automatically selected, or by choosing the handset from the touch panel interface. If this does not work when in a call, and the far end does not hear the near end, then you are seeing a hardware failure. Here is what it says the admin guide.
"The handset is automatically selected when you lift it off the hook. Only the microphone in the handset is enabled. Neither the internal microphone nor an external microphone can be used."
You might need to open a TAC case to get the part replaced.
HTH
