Brother MFC4820C Service Manual page 219

Selectors 5 and 6: Busy tone detection in automatic sending mode
These selectors determine whether or not the equipment 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 equipment does not disconnect the line.
Setting selectors 5 and 6 to "0" and "1," respectively, makes the equipment detect a busy tone only
after dialing and disconnect the line.
Setting both of selectors 5 and 6 to "1" makes the equipment 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 equipment automatically disconnects the line upon
detection of a busy tone in automatic receiving mode.
App. 4-7