Had this happened to our customer's a few times. They were using the guest mailboxes with notification. Check your open loop interval for disconnects on all co lines. Make sure when the system hangs up a call the line disconnects. Check the analog ports for outdial and lock them down. Call ESI tech support as we informed them of this a few times and I know they had a tech write up about it. Lastly, download PABX soft and install it on a temporary basis to track all of the call patterns. This will show what is up.