Brother MFC6800 Service Manual page 167

Selectors 5 and 6: Busy tone detection in automatic sending mode
These selectors determine whether or not the machine automatically disconnects a line upon
detection of a busy tone in automatic sending mode.
Setting selector 6 to "0" ignores a busy tone so that the machine does not disconnect the line.
Setting selectors 5 and 6 to "0" and "1," respectively, makes the machine detect a busy tone only
after dialing and disconnect the line.
Setting both of selectors 5 and 6 to "1" makes the machine detect a busy tone before and after
dialing and then disconnect the line.
Selector 7: Busy tone detection in automatic receiving mode
This selector determines whether or not the machine automatically disconnects a line upon
detection of a busy tone in automatic receiving mode.
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