Fax issues typically boil down to connectivity. Get a good quality phone cable, get it straight between the printer and the wall, check for a CLEAN dial-tone, free of static and crackling. Once you get that far, try a test-fax using the scanner bed.
Once you have THAT working, then you can move gradually into re-adding your existing hardware into the mix. Reattach that magical 20 year old splitter you have on the wall, see what happens when you test-fax again. Add that existing phone to the splitter, see if you can still test-fax. You can then try faxing from the PC in there's one connected.
ANY other error would be a problem that can be communicated via the front screen or an error window on a connected PC. Out of Ink? Printer not found? Paper Jam? Fax not configured?
I hope this helps. I did the best I could with the info I was given. :)
PS...need more concrete info on the question.