Brother FAX-2440C - Manual (Page 184)

APPENDIX A A - 12
When you are using a TAD, the LCD shows Ext. Tel in Use .
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, do not connect a
TAD elsewhere on the same phone line.
TAD
TAD