JK Audio THAT-2 - Manual (Page 5)

5
Operation
The Handset / Line In button selects which signal will
be sent down the phone line; the handset
microphone, or the line input jack.
When the Handset / Line In button is out, you can use
the handset to speak and listen as you normally would.
The sound that you hear on the handset is always
available on the From Phone output jack.
With the button in, the handset mic is disconnected and
the line input is connected to send audio signals down
the phone line.
You can also use the Handset / Line In button to turn off
the handset mic if you are listening to the line output
signal over a loudspeaker. This will prevent feedback
between the handset microphone and loudspeaker. In
any case you can always listen over the handset
receiver.
We designed this interface to work with most popular
telephone designs. If a conflict arises it will typically be
on the line input side. If you attempt to send voice band
signals down the line input jack and the distant party
complains that the level is too loud or severely distorted
(clipping), try to lower the transmit signal on your audio
equipment. If this does not help, discontinue use
immediately.