No errors in Alarm History or PRI Diagnostics reports?

If it's happening 6-8 times over three years is it mostly recently or spread out like once every four months which would be brutal to diagnose?

If it happens more frequently, like once a day, ESI can turn on PRI debug which would show what the telco and the 600 are doing at the start of a call. The problem is that can eat-up debug space quickly.

You probably have also checked that the telco has both circuits provisioned for NI-2, with FAS. You would probably have worse problems if either of those weren't the case.

-Paul
Alt Systems, Inc.
Buffalo, NY