It seems to be very random, say once a month, so I'm not usually running the monitor when it happens. I find out later when I see it has been in the clearing state for several days. We do have and internal ATM4 on the same system that does not exhibit this behavior. So I am less inclined to think it's a phone provider issue. I'm wondering if there are some config tweaks I can do.
It also seems that it happens to all of the trunks (we're using 4) at around the same time.