Would not go that far yet.
It is still beneficial to know if the sending side is getting an error or an OK signal. Your last post simply indicated the issue again, but not how the sending side perceived the completion or lack thereof.
If an OK, then I would assume that the sending side is seeing the OK from the 3CX system and therefore the 3CX system is likely having an issue. If not an OK, then it could be the Patton or the 3CX system and possibly even the FXO line, but this seems unlikely given the receptiveness that seemingly cuts the fax off at 40 pages.
While I do know that the 3CX side does have a call duration limitation, the fax calls being 25 minutes fall well within the 3CX setting. What I do not know is if the 3CX side has a limitation on fax pages associated to the conversion to PDF.
I do not use a similar configuration as I use SIP, but I will try and send a large fax and see if I have the same issue.
Most folks who bring a fax line in, as you have done, simply take the line all the way to a fax machine and avoid the intervening conversions and potential issues.