Brother FAX 1840C - Manual (Page 182)

APPENDIX A A - 10
Connecting an external device to your
machine
Connecting an external telephone
You can connect a separate telephone (or telephone answering
device) directly to your machine as shown in the diagram below.
Connect the telephone line cord to the jack labeled EXT. in the back
of the machine.
When you are using a TAD, the LCD shows Telephone
(MFC-3240C and MFC-3340CN only) or Ext. Tel in Use
(FAX-1840C and FAX-1940CN only).
Extension
Telephone
External
Telephone