LG -AD5435 User Manual page 17

Call Volume(During a call : [MENU]+[1] )
You can control call volume during a call
In Call Memo(During a call : [MENU]+[2] )
You can save memo during a call and this data is saved at call history
Mute(During a call : [MENU]+[3] )
Sometimes it is necessary to mute a call so that caller does not hear you or certain
background noise. To use this function during a call, Press Soft Key 1 [Option] and [3].
If you need to release mute press the Soft Key 1 [Option] and [3] key again.