Sounds as if you have a Disconnect Supervision issue.
Here's an easy test:
1)Assign one of the GATM lines to a set.
2)Call the set on that line.
3)Have the set user put you on hold.
4)Disconnect the call and see how long it takes for the flashing indicator to disappear. It should be nearly immediate. Some C.O.'s can take up to 12 seconds.
If it doesn't disappear, Telco isn't providing Disconnect Supervison.
And of course, your GATM trunks must also be set to Super.
-SD-