Brother MFC-420CN - Manual (Page 155)

APPENDIX A A - 9
Connecting an external device to your
MFC
Connecting an external telephone
You can connect a separate telephone (or telephone answering
device) directly to your MFC as shown in the diagram below.
Connect the telephone line cord to the jack labeled EXT. in the left of
the MFC.
When you are using a TAD, the LCD shows Ext. Tel in Use .
Extension
Telephone
External
Telephone