I feel your pain.
Been there, done that - many times.
The Adtran router is there because Windstream is delivering the voice via VoIP to the router. The router hands off to a TDM PRI.
Generally speaking, you can blame the Adtran.
If when the problem happens, does the carrier say they don't see a problem?
Here's what I've done in the past.
When the T1 goes down, I've "accidentally" power cycled the Adtran.
When it magically started working again, I called the carrier and demanded they replace the router.
Ralph