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.
EDIT
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.
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.