Ok, let's look at what we have narrowed down.
1 - The issue does not happen on other trunk card/types in the same phone system - i.e. 4COT card w/ analog pots line
2 - The problem only happens to certain LD numbers, not all, but the same results with those numbers
3 - The problem only happens on the PRI circuit
4 - The problem happens regardless of LCR patterns - eliminates lcr programming
Taking all this into consideration, this is starting to not even sound PRI related but more like carrier translations.
Does the problem occur always on the same numbers? Meaning is it wishy/washy or does it happen every time on particular numbers that do this?
What area codes are they? Is it always the same area code(s)?
Is the pots line carrier the same as the PRI carrier?
Can the carrier send a tech and place these calls from the building dmarc using a tester unit and see if the ringback problem persists? If so, they need to address it and this proves its not your pbx.
-Phil Ingram
{All-You-Can-Eat Telecom and IT Buffet}