We have around 100 Asterisk servers all using Pika for faxing. Each server sends and receives anywhere from 20 to 200+ faxes a day. Every system uses analog, T1, or PRI lines. Overall, reliability is high with all of them (>95% success rate). However, about once a week we will get a call from a customer that is having problems with reliability. I am at a loss on how to properly (and efficiently) diagnose the problem.

Typically, the first thing I will do is call the number and verify that there is indeed a fax machine is on the other end. At the same time I will note how long it takes the machine to pick up (and adjust timeout settings if needed). Next I will either record the call and look at look at it or dial out on the individual lines to see if there is any sort of interference or static. After that, I more or less fumble around resending the fax many times adjusting things like the modem type and echo suppression delay. Sometimes I get lucky and other times I don't.

Any advice or recommended plan of attack?