Geemarc CL400 - Manual (Page 33)

Note: When the tone (treble), earpiece volume and
sending levels are set to the maximum, whistling
may appear. If whistling occurs the sending level
settings should be decreased (sending level switch
located on the right side). When the switch
located behind the phone is ON, the extra-receiving
and tone functions are automatically activated
each time you use the phone.When the switch
is switched OFF, you will need to activate the
function by pressing the button
.
33
RECEIVING TONE AND VOLUME
TONE
Bass
+10db max
Treble
+10db max
Normal
VOLUME
0dB 15dB OFF
15dB 30dB ON