When using automatic gain control (AGC), the phone tries to maintain a constant audio level even if the incoming call changes between loud and quiet.
1.Press the 
button.
2.Use the up and down cursor keys to highlight Options & Settings. Press Select.
3.Use the up and down cursor keys to highlight Advanced Options. Press Select.
2.Use the up and down cursor keys to highlight Automatic Gain Control. Press Select.
•Use the up and down cursor keys to switch between Handset, Headset, or Speaker settings for AGC.
•To change the highlighted setting, press Change.
4.When completed, press Save.
5.Press Back / Exit to exit the menus.