With a call to an extension on your system, when the extension hangs up it is the trigger for the system to "clear down the call, freeing up the equipment for the next call. If there is no clear down signal on your lines (not common on PSTN lines but there are some that do like reversal on idle) then you are connecting two users together, neither of which will trigger the cleardown. So the call will stay up indefinitely when this happens you basically have to pull the card out of the system and then put it back to free up the circuits.
If you can find out if you have or can get something like reversal on idle then it may be worth looking at the programming. Feel free to post back if so.