LG G3100 User Manual page 196

To answer an incoming call, simply press the [Send] key.
The handset is also able to warn you of an incoming call while you
are already in a call. A tone sounds in the earpiece, and the display
will show that a second call is waiting.
If Call Waiting is on, you can put the first call on hold and answer the
second, by pressing [Send] or by selecting the [Answer] key.
You can end the current call and answer the waiting call by selecting
the [Menu] key, then .
You can reject an incoming call without answering by simply
pressing the [End] key.
When in a call you can reject an incoming call (1st call) by pressing
the [Menu] key and selecting or by
pressing [End] key.
You can control the volume during a call by pressing [Options] key
followed by U or D .
and selecting
91