Brother MFC-3420C - Manual (Page 135)

USING BROTHER PC-FAX SOFTWARE 6 - 23
6
Enter the sender information, and then click Next.
7
Select Brother MFC-XXXX (Where XXXX is your model name)
in the Please select the fax device pull-down list. Click to
select the Enable Receive check box. Specify the number of
rings before your machine will attempt to answer the fax and
click Next.