The triggers approach is interesting, but it blindly adds the additional time to the last entry. That's incorrect if there are multiple calls as the last one started gets all the time. Anyway I just uploaded a daemon monitor program that uses dbus to get the call id and update end_time. It has a GUI to display the call summary, though the plan minutes is only implemented to match my carriers minute rules.
http://maemo.org/packages/view/minutemonitor/