I have a multifunction fax/scanner/printer at home, and I send faxes just once or twice a year. Obviously, I'm unfamiliar with the process. I tried to send my fax three times yesterday, and each time the send failed. I had no idea why. Looking for help I found JustFax Online and decided to use the service. JustFax Online's first attempt also failed! However, JFO reported that the reason was that the receiver's fax was engaged. JFO automatically re-tried after 6 hours. Success! I would not have thought to delay my re-try, and would have simply thought I was doing something wrong.
Thank you, JustFax Online. You were worth the fee.