From the Norstar Alarm an d Event Code Manual:
40 The long term alarm threshold has been surpassed in the DTI for the Unavailable Seconds Error. Most likely an irregularity with the PSTN connections. Check the logs and look for events ranging from 315-336. If this alarm occurs more than once over a two-week period, contact your local support group.
Please check your logs on site and post them. It certainly appears from Norstar advice that this is a PSTN problem, not one you caused. You really do need the extra events log to get a clue as to what is happening.
If you put one of those dial tones straight from the channel bank into a single line phone (or clip on with a butt set), do you get the same problem? If so, it has to be the PSTN or the channel bank.
Its easy enough to have the carrier switch a channel bank.
All you need is a little detective work. Document as much as you can from the logs and the single line phone test and let us know what happened.
You can get the Norstar Alarm and Events codes from:
https://www.nev-comm.com/NorstarAlarmCodesEvents.htm Awful handy to have around if you print it out and keep it for when you need it, like now.
Also it has no mention of "Normal Call Cleared"
What CSU/DSU device are you using, Adtran? Name and model number? Have you called their tech support?
