AT&T 8102 User Manual page 37

Switching from the Headset to the Handset
If you are on a call on the headset and want to switch to the handset,
you can do so by lifting the handset. The call is now on the handset.
Press the Off button on the headset adapter in order to turn off the light
next to the On/Quiet button.
To switch from the handset back to the headset, press the On/Quiet
button and place the handset back in the telephone cradle.
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