Communication error when changing fax cables

[Equalata]

I've searched these forums for hours and can't seem to find a solution. I'll put up a summary of the events and see if anyone has any more suggestions - thank you in advance. 

My clinic needs to move our printer to another spot. The cable that comes with the printer is too short, so we bought another longer one from JBHiFi. Turns out HP printers need the 6p2c jack on the fax cable (with the 2 wires) and we'd bought the wrong one, even though it's standard to have 4 wires now, and the techs said it shouldn't matter anyway. But no faxes are being sent or received, I'm just getting Communication Error 344 (with the 6p4c cable) and 345 (with the long 6p2c cable).

I tried to contact HP after finding that the 2c variants are really hard to get. Best to get the official cable, right? They directed me to an authorised seller who had no idea what I was talking about and directed me right back to where I started, at a specialised electronics store Jaycar. Another post on here said they will make one for me, but they said they don't do that.

Finally I found one on Ebay - the only 2c I could get, an RJ12 6p2c. It's the only Ebay store that had one and doesn't do returns. I excitedly tested it and guess what...no dice. 

What I've tried:

- Turning/unplugging off and on

- Disabling ECM

- Changing the fax speed to slow

- Disabling fax and re-enabling it including setup

If I carry it back to its original spot and use the original cable, the faxes work fine. So it must be the cable! But the machine can't stay there because we need that space for a new employee.

I can't keep buying new cables either, they're all in their plastic packaging that you have to cut to open, and I can't return the one to Ebay. I've wasted hours and hours learning about all the types of cables, and trawling through the forums and losing my mind. Why has HP made this so difficult, and how can I proceed? All I need is a longer cable! 

Thanks

Edit: I've tried sending to multiple numbers, not just one.