Brother MFC-420CN - Manual (Page 157)

APPENDIX A A - 11
When you are using a TAD, the LCD shows Telephone .
If You Do Not Subscribe to Distinctive Ring Service:
You must plug your TAD into the EXT. jack of your MFC. If your
TAD is plugged into a wall jack, both your MFC 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