Brother FAX-1840CN - Manual (Page 174)

APPENDIX A A - 5
Connecting an external device to your
machine
Connecting an external telephone
You can connect a separate telephone as shown in the diagram
below.
When you are using an external or extension telephone, the LCD
shows
Telephone (FAX-1835C, MFC-3240C and MFC-3340CN
only) or Ext. Tel in Use (FAX-1840C and FAX-1940CN only).
Extension
Telephone
External
Telephone