Brother MFC-820CW - Manual (Page 26)

Chapter 1
10
Note
When you have an external Telephone
Answering Device (TAD) on your fax line,
your TAD will decide the number of rings
before the call is answered.
ECM (Error Correction Mode)
1
The Error Correction Mode (ECM) is a way for
the machine to check a fax transmission
while it is in progress. If the machine 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
machine.