One option may be to use a SetSessionInfo step to set a call context variable once the caller has made their selection. You could then look at using either the built in "Call Custom Variables" report, a custom report, or a SQL query to view the data depending on how you need it presented.

If you'll be queuing to the departments another option may be to separate the IVR and department logic into separate "applications" if you haven't already done so. In this type of setup you configure a separate application, and associated trigger, for the IVR and each department. You then use Redirect steps within the IVR to transfer callers to the trigger for the appropriate department. This would allow you to use standard application reports to determine call counts for each department and avoid having to work with custom variables. While this method may be easier from a reporting standpoint it does introduce additional complexity from a configuration standpoint.