Brother DCP-L2520DW Basic User's Manual page 55

you answer an incoming fax on the telephone,
you can make the machine receive it by
pressing * 5 1, or by pressing Fax Start, and
then choosing Receive. If you answer an
incoming fax on an extension telephone that
shares the line with the Brother machine, you
can transfer the fax to the machine by
pressing * 5 1.
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