Sensitivity in sending direction can be adjusted in 9 steps

 for handsfree microphone and for handset microphone.

Therefore you can choose following hidden page on the web interface of the telephone:

--> open web interface of telephone
--> login with admin account
--> http://<IPadress telephone>/welcome.html  change  into:  http://<IPadress telephone>/voiceadj.html

There are following adjustment parameter:

HandFree MIC TX:  actual value,   -->   adjustments:  0-9

HandSet   MIC TX:   actual value,   -->  adjustments:  0-9

Handset   AEC:                   1             -->   0:Disable,  1: Enable


Increasing the value increases the microphone loudness.
As default now the Automatic Eco-Cancelling (AEC) is activated.  AEC should deactivate or minimize an Echo to the earpeace.  If there is no improvement or Echo is too high, you should reduce microphone level again.