I think I know what is happening, not sure how to fix it yet.
There are 2 sets of trunks, 3 trunks with one provider and 1 trunk with a second provider. If I disable either trunk set, the other set will register and work fine. Its when both trunk sets are enabled that the single trunk set will drop. It should be noted that when the trunk drops, tcpdump shows no outgoing traffic to the provider of that trunk on the wan interface; however the lan interface shows the traffic coming into the box.
So at least I can replicate the issue by enabling/disabling the asterisk trunks without rebooting the pfsense box each time.
I suspect that this vanished trunk registration traffic is being incorrectly routed to the other provider and thats why I don't see it. I also have no idea why it would work initially for a bit when the trunks are all enabled and then it dies with no trace.