If you are in the EU then there is no disconnect clear signalling on alog trunks.
The only thing what can be done is to check wether the busy tone of the alog lines is recognized by IP Office.
Without disconnect clear IP Office continously listen on alog trunks for busy tone, if the busy tone is on the line with a minimum duration of 4 seconds then IPO will drop the line.
In the sysmon traces of the alog lines you can see wether the busy tone is recognized or not, if not you get messages stating "sloppy busy tone detected" and IPO resets the timer for a reliable busy tone.