Hi there. The Printer Doctor is in and ready to help you!
While you say this setup has worked for a while prior to now, I don't think it a good thing to have 2 fax machines on one line because:
1) If they're on the same line with the same number, only one can occupy/access/use the line at any given time. If one is sending, the other cannot receive unless the other drops use of the line.
2) If after no connection on the send fax, it will time-out and then (usually) try to connect again in a few seconds. So that the SEND data for the fax is still in the queue to try again. That can cause conflict on the line, and the Receive fax thinks it's getting a call, but cannot ring because the line is not actually released by the Sending fax. This is kind of like dialing your own home phone number from your home phone handset--you get a busy signal, but the phone cant ring because you're already on the line.
So what you end up with is two trains running on the same track in opposite directions. Here is what I suggest:
Change your setup by using one fax machine to both send and receive, OR
Get a second dedicated fax line fro the other fax machine, OR
If you want to keep the setup you have, turn the auto-receive OFF only when you are sending a fax from the other machine. Once it is completed, you can switch the other back to auto-receive.
Finally, if you don't want to add a second line, it may be cheaper to subscribe to one of the many online faxing services, e.g.MyFax.com.
Which choice you make will depend on how many faxes you send/receive daily, the size of the fax files, etc. The online fax service is great because you won't have to worry about large documents tying up the memory on the fax, and you cn send everything to a folder and print it out immediately or later, plus always have an electronic record as well as a paper copy.
Of course, for sending efficiency, everything you send via the online fax service will need to be in electronic file form--just like an email attachment. So, you could use the online service for receive-only, and send your hard-copy documents from the Officejet Pro 8500a.
If my instructions lead you to the solution of your problem, please remember to click the green ACCEPT button (even if you have a subscription) so I will get credit for helping you today. If you feel I've serviced you very well, positive feedback and a bonus is also greatly appreciated.
If your problem is not solved, instead of giving negative feedback please give me a chance to further assist you.
For future problems, you can ask me a question directly by going to my page at http://www.justanswer.com/electronics/expert-printerdr/
Thank you, and HAPPY PRINTING and FAXING from the Printer Doctor!
Not satisfied with answer. Have this setup working in many offices for many years. Why was my account billed?
Have been a phone tech for 20 plus years and am not an idiot. If these offices could afford multiple fax lines, they would have a dedicated line for every device. Not sure if you understand the real world of small business.
Sorry you feel offended. My intent was to offer alternatives. I will opt-out and release this question so that another expert may be able to help you with what you want.