I recently discovered that I have had an issue with an atlas 550 utilized as a channel bank where slot 2 was somehow slightly corrupting modem type calls going through it. I have a pri serving 3 FXS-8 cards and have a looped dual t1 card in slot 1 providing ANI and DNIS info to the receiving equipment attached to the FXS cards. The FXS card in slot 2 is configured for circular call routing. I created a test route to slot 3 port 1 with all the same configuration as as slot 2 ports 1-8 to allow me to isolate a DNIS for testing purposes.
From a particular location I was having communication issues with I routed the call to slot 3 port 1 so I could listen in. When I did that everything worked fine. so I sent it back to the normal slot 2 group and it stopped working again. After much testing and comparing making sure there were no setting or programming differences between the two paths and redirecting slot 3 port 1 to the exact same receiving device that fails when it gets the call from slot 2 I determined there was an issue with slot 2 or its FXS. I swapped slot 2 and slot 3 FXS modules and still the issues remained at slot 2. Last I backed up the config file and loaded it to a backup 550 and brought that 550 live assuming all connections and workload of the original unit. Slot 2 on the replacement unit no longer had communication issues with that that site. So I determined that there something is wrong with the chasis on the original unit.
Its important to note that the large majority of other locations were sending what would be considered identical traffic through identical routes (minus the differences in routing by the phone company network) to the slot 2 group without issue and the exact number is unknown but it is a small percentage of locations that may have been failing because of the slot 2 issue that was discovered. Had it not been for repeatable success on slot 3 port 1 I would have assumed this was a telco issue.
I spoke with Patrick at Atlas and since i'm out of warranty I now have to return the unit to Adtran pay for testing and they may or may not find something to repair. The most frustrating part of this is wondering how many times I have blamed the phone company for something that was wrong with my Adtran all along, possibly since I bought it.