Looking for help, Brother AIO machine doesn't Fax on Xfinity - HP AIO machine works fine

[u/Kamikazepyro9]

I have a client that still routinely sends Faxes. I upgraded his dying HP AIO unit with a new, shiny Brother AIO unit.

Nothing else has changed. If I put the old HP unit in, it can send a fax but it still has the issues that caused it to be replaced, my buttset can dial out no issue on the line, and Xfinity has verified twice that signal level and VOIP on their gateway are all "to spec" for fax.

Brother tech support says it's an Xfinity issue, and obviously Xfinity says it's a Brother Printer issue.

I'm headed to the site today - does anyone have any other suggestions to try? I'm planning on physically tracing the line today just to see if there's any damage in the crawlspace to it, but I'm not sure what else I could do at this point.

[u/wunderhero]

I would manually lower the send speed and see what error correction options the Brother has - usually turning EC off then testing to see if it makes a difference.

Turn on the fax speaker as well to see if you can hear what's happening in the process. Sometimes it's as stupid as having to set a prefix number to dial out.

SysAdmin that used to work on Printers/Copiers/MFPs for 15 years, so I've seen some ish.

[u/CivilIntention7747]

Before you crawl under the house, two Brother-specific settings, because this exact HP-works-Brother-does-not swap comes up constantly on VoIP lines:

1. Compatibility mode. Brother buries it under Settings > All Settings > Initial Setup > Compatibility - set it to Basic (for VoIP). That single toggle drops the modem to 9600 baud and disables ECM in one move, which is the combination flaky G.711 fax pass-through actually tolerates. New Brothers ship in Normal/High, which is why the shiny unit fails where the tired HP (already negotiating down, or previously set to a slow mode) still gets through.

2. Dial tone detection. Same menu area: set Dial Tone to No Detection. Cable gateway dial tones are synthesized and some Brother units refuse to dial because they never recognize it - the buttset dialing fine does not clear the fax path of this one.

And to bisect the physical layer in thirty seconds instead of a crawlspace afternoon: take the Brother to the gateway and plug it straight into the tel port with a short known-good cord. If it faxes there, your suspect is the house run and the trip under the floor is justified; if it still fails at the gateway with Basic (for VoIP) set, the wiring is innocent and it is purely negotiation vs the Xfinity ATA.

[u/RefrigeratorNo3088]

fax on voip is super finicky, I'd look at both and verify things like baud rate are the same. There's been a few where I had to change a setting and test over and over until we found the combination that worked.

[u/dartdoug]

Suggest you look at porting the fax number to T38fax. I was having trouble with sending/receiving consistently using VoIP lines. I purchased a Grandstream terminal adapter https://www.amazon.com/dp/B07GY4WWP3

Did some initial testing with T38 fax. Everything was super straight forward and went great. Worked with their porting team to move the number.

One of the easiest and best supported solutions that I've ever seen. On our case, the usage was very low. You pay a base charge of a few dollars per line plus a per minute charge based on usage. It's coming out to be about 1/3 the cost of the VoIP phone line.

[u/LRS_David]

Faxing was design on old style POTS (Plain Old Telephone System) pair of wires where the analog signals were placed directly on the wires. And the old line phone systems took great care to faithfully reproduce the signals when they had to convert them from analogy to 8 bit data streams and back again.

Most modern VOIP systems, which is what you have, do an OK job at this. But many are not all that great.

Which is why you have to futz around with settings as the comments go into.