LG G3100 User Manual page 197

You can mute the microphone during a call by pressing the [Options]
key then selecting . The handset can be unmuted by pressing
the [Unmute].
When the handset is muted, the caller cannot hear you, but you can
still hear them.
To turn DTMF tones on during a call, for example, to allow your
handset to make use of an automated switchboard, select the
[Options] key, then . DTMF tones can be turned off the
same way.
The Messages and SIM Tool kit main menu options can be reached
from the in-call menu by selecting the [Options] key, then
(Screen name depends on the network operator's
SIM). SIM Tool kit menu is shown only when SIM in the handset
supports SIM Tool kit service.
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