IOGear GBHFK331 - Manual (Page 24)

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Re-Dailing Short press the MFB button twice under standby mode.
Adjusting the
Volume Press the “+” or “-“ button repeatedly until you reach the
desired volume level.
Mute* Press the MODE button for 3 second during a call to mute
and the RED LED will turn on.
Un-Mute Press the MODE button during mute mode to un-mute the
Speakerphone and the RED LED will turn off.
*Mute mode means mute the microphone. The caller or call receive party
will not be able to hear anything conversation from Speakerphone side.