Canon imageRUNNER ADVANCE C3500 III Series Service Manual page 1188

3. The number printed following colon (:) in
"COMM.MODE" field on FAX ACTIVITY REPORT
TX/RX shows line type used for sending/
receiving.
E.g. "ECM:2" => Line 2
NOTE:
If E744-5000 error code (Fax software version mismatch
error) occurred while sending or receiving fax, upgrade the
firmware of 2-line Fax to the latest version.
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9. Installation