We have seen similar events and suspect that someone is calling a number that in some way is not sending back the information to tear down the call completely when they hang up. We have not been able to prove this yet, not knowing the number being called. In our case it is always a local call. The same channels when reset work just fine in subesquent testing.
Not much help in solving your issue, but sometimes its good to know you are not the only one out there seeing these problems!!