Brother MFC-3420C - Manual (Page 30)

INTRODUCTION 1 - 9
ECM (Error Correction Mode)
The Error Correction Mode (ECM) is a way for the MFC to check a
fax transmission while it is in progress. If the MFC detects errors
during fax transmission, it resends the pages of the fax that had an
error. ECM transmissions are only possible between fax machines
that both have this feature. If they do, you may send and receive fax
messages that are continuously being checked by the MFC.
Your MFC must have enough memory for this feature to work.
Connecting the MFC
Connecting an external telephone
If your MFC does not have a handset, you can connect a separate
telephone (or telephone answering device) directly to your MFC as
shown in the diagram below.
Connect the modular plug on the telephone line cord to the jack
labeled EXT. in the back of the MFC.
When this phone (or TAD) is in use, the LCD shows Telephone .
Extension
Telephone
External
Telephone