Brother MFC-640CW User Manual page 183

The TAD must answer within four rings (the recommended setting is
two rings). The machine cannot hear CNG tones until the TAD has
answered the call, and with four rings there are only 8 to 10 seconds
of CNG tones left for the fax "handshake". Make sure you carefully
follow the instructions in this guide for recording your outgoing
message. We do not recommend using the toll saver feature on your
external answering machine if it exceeds five rings.
If you do not receive all your faxes, reduce the Ring Delay
setting on your external TAD.
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 your
machine, and use that number as a fax number. The
recommended setting is at least four rings on the external TAD
when you have the telephone company's Distinctive Ring
Service. You cannot use the Toll Saver setting.
If You Do Not Subscribe to Distinctive Ring Service:
You must plug your TAD into the EXT. jack of your machine. If
your TAD is plugged into a wall jack, both your machine and the
TAD will try to control the phone line. (See illustration below.)
Unless you are using Distinctive Ring, the TAD must be
connected to the EXT jack of the machine.
When you are using a TAD, the LCD shows Ext. Tel in Use.
TAD
TAD
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