Brother all-in-one fax fails over Xfinity VoIP while the older HP works

If the Brother remains unreliable, a dedicated fax-over-IP service using T.38 may be a better long-term option than sending fax tones through a regular VoIP line. A compatible analog telephone adapter can connect an existing fax machine to that service, and pricing can be reasonable when fax usage is low.

There are two Brother settings worth checking before tracing the cable. Set Compatibility to Basic or the VoIP-oriented mode; this typically lowers the modem speed and disables ECM together. Also set Dial Tone Detection to No Detection, since some cable gateways use a synthesized dial tone that the fax doesn’t recognize. For a quick wiring test, move the Brother next to the gateway and connect it directly to the phone port with a short, known-good cable. If it works there, investigate the house wiring. If it still fails, the problem is likely the fax settings or how the unit negotiates with the gateway.

Faxing over VoIP can be extremely sensitive to the combination of baud rate, error correction, compression, and gateway behavior. Try changing one setting at a time and retesting rather than assuming that a working buttset proves the fax path is compatible. The older HP may simply be falling back to settings that the Brother is not using automatically.

Start by lowering the fax transmission speed and experimenting with error correction. On many Brother units, disabling ECM and using a slower speed—often 9600 baud—makes faxing over a VoIP gateway much more reliable. Turn on the fax speaker too, so you can hear whether the call is failing during dialing, negotiation, or transmission. Also check whether the machine needs a dialing prefix.

Traditional fax was designed for analog telephone lines, while VoIP has to convert those tones into data and back again. Even when the gateway reports that everything is within specification, fax reliability can vary between devices. That’s why slower speeds, disabled ECM, and a direct connection test are usually the most useful next steps.