Brother 575 User Manual page 25

If you subscribe to your telephone company's Distinctive Ring
service:
You may connect an external TAD to a separate wall jack only
if you subscribe to your telephone company's Distinctive Ring
service, have registered the distinctive ring pattern on the fax
machine, use that number as a fax number and set your fax
machine's Receive mode to MANUAL. The recommended
setting is four rings on the external TAD when you have the
telephone company's Distinctive Ring Service.
If you do not subscribe to Distinctive Ring service:
You must plug your TAD into the EXT. jack of the fax machine.
If your TAD is plugged into a wall jack, both your fax machine
and the TAD will try to control the phone line. (See illustration
on page 1-10.)
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Introduction