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I'm not a phone guy at all, but I have a strange problem. One of my clients' staff members has several fax machines on the same line (with staggered answering on number of rings for failover). I believe they dial out on whatever rollover line is available, but I'm not positive. I can certainly clarify this if required.

Anyways, their Windows Fax Service users and their Multi-function Printer fax users are experiencing an intermittent issue where they're manually keying in a number on the fax machine (as well as typing it into Outlook for the Windows fax users) and it's going to a different number, as in the "wrong" number people are calling back and saying, "why did you fax this to us?". Now it's not just any number, it's going to another regularly faxed destination, just not the right one.

So I'm wondering if two machines are trying to fax at the same time and getting "the lines crossed".

Is this possible that there's some sort of cross-talk or "routing" issue? Again, absolute dullard when it comes to POTS.


Found out that Sally wants to fax to 555-1234, but it's going to 555-5678. At roughly the same time, Bob has been getting a "failed" fax response when trying to fax to 555-5678, so there's definitely some "cross-over/cross-talk" happening.

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Are these machines plugged into a PBX (or a Centrex line)? Can you retrieve anything from the PBX logs that might shed some light on where the dialed number is coming from? – voretaq7 Sep 15 '11 at 18:09
@voretaq7: they use a PBX there, yes; it's analog as far as I know. Found out that when staff A faxed to number B, staff B had a fax fail destined for number B, so there's definitely some sort of "crossed lines" happening. – gravyface Sep 15 '11 at 18:19
I blame the PBX (absent anything else to point at) -- If it's an old analog PBX debugging will be painful, but if the problem is what I suspect it should also be extremely infrequent (probably a race condition of some kind, requires two numbers dialed at the same instant to screw it up) – voretaq7 Sep 15 '11 at 18:21
When you say they're on the same line, what do you mean? A POTS line can only have one phone call at a time. Do you mean one telephone number? If you actually mean one line, then of course multiple things attempting to dial on the same line are going to break. – derobert Sep 15 '11 at 20:17
@derobert: I believe outbound calls use whatever line is available to dial out. Incoming calls come in on one line; the fax devices all are set to answer at different number of rings to provide some redundancy. – gravyface Sep 15 '11 at 21:38

Check to ensure the fax lines are not all assigned to the same analog PBX line card, at least not in a configuration where multiple fax lines can be in use at the same time. Another possible consideration is to ensure the PBX analog line cards are rated to support x-number of simultaneous fax sessions as a lot of PBX analog cards are not specifically designed to keep up with the demands of faxing across all their ports simultaneously.

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Telco and contractor who manages the phone system are looking into it, so we'll see. – gravyface Sep 17 '11 at 12:41

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