Busy 1 4 27, will busy the first 16 VM ports, 1 4 28 will get the rest, if it comes across a "busy" port courtesy down it until you can "resource" port and see if it has a terminating device, if not that's the culprit. If it does, you'll have to flip the coin if it shows a trunk, then it could be the issue or somebody still listening to the greeting, if it shows a user, call them to see if they're busy.