Huge apologies here. I should have updated when I found a solution to my problem a long time ago.
I'm doing so now to provide closure.
It turns out the only time I had the problem was if the T1 was configured as ESF.
Not all the time, not every T1. Sometimes it would go for months without a problem and then it would rear its ugly head again.
It only happened with this one specific carrier.
The solution was to convert the trunks to SF. Problem solved.
I've no idea why it happened. I'm just glad I found a fix.