The most critical for faxing over VoIP is turning off ECM (error correction mode). Most newer fax machines have ECM capability, which analyzes the received frames and detects any corrupted data. Noise, poor signal strength or packet loss can cause part of the the fax information to be lost during transmission. If any is detected, then a retransmit signal is sent until a error free frame is received.
The problem with this setting over VoIP is that some packet loss, latency or jitter will usually exist on an Internet connection. The retransmits tend to increase the call duration, adding to the instability of the fax. When packet loss gets to a 1 or 2 percent level then faxes will fail consistently. By disabling ECM, the receiving fax machine will not continually request any slightly corrupted transmissions to be resent, ensuring a greater chance of a completed fax. This could result in a fax image that is of slightly poorer quality, but in most cases it should be more than sufficient.
How to Disable ECM on most HP Fax machines:
These settings should work closely for most HP fax machines. If you do discover any slight differences, you should look to locate the ECM setting in the Advanced Features Setup menu.
For most HP 7000 model Officejets. Press Setup Press the down arrow until you get to Advanced Fax Setup. Then hit OK. Press the down arrow until Error Correction Mode displays. Then hit OK. Press the right arrow until OFF displays. Then hit OK.
For most HP 2000, 4100, 5000, 6000 & 6100 model Officejets.
From the Front Panel Press Menu. Hit the Right Arrow until you get Advanced Fax Setup. Then hit Enter. Hit the Right Arrow until you get Error Correction Mode. Then hit Enter. Hit the Right Arrow for Error Correction Mode Off. Hit Enter.
How to Disable ECM on some Canon Fax machines.
For Canon models B115, B120 & B140 fax machines.
Press Function. Press Start/Copy. Hit the Right Arrow until TX Settings appears. Press Start/Copy. Hit the Right Arrow until ECM TX appears. Hit Start/Copy to turn it OFF. Note: By hitting Function at any time will return you to the previous level.
For Canon Fax Phone B95.
Press Function. Press Start/Copy. Hit the Right Arrow until TX Settings appears. Press Start/Copy. Hit the Right Arrow until ECM TX appears. Hit Start/Copy to turn it OFF. Hit Function to return you previous level until you can choose ECM RX by hitting the Right Arrow. Press Start/Copy to turn it OFF.