I've found some machines will not work with each other and the fax does not go through. You may find folk that DEMAND AN ANSWER but the simple truth is "it does not always work."
I have an epson workforce 630. I am using it to temporarily replace our main office copier/fax machine until its repaired. Initially I could not send or receive faxes, but then I was instructed to turn off Error Correcting Mode (ECM) and turn off v.3.4. After I did this I was able to receive faxes and I was able to send a test fax to epson which they received, but when I try to fax to other numbers it does not go through. Any ideas as to why this might happen?