For a quick view of costly calls use these reports (dialled numbers covered for privacy:
Top 25 High Cost Numbers
This looks at all the numbers dialled for that period, then collates and sorts them in descending order.
If you see a lot of calls to a particular number, you may want a bit more information such as the extension(s) making the calls. Highlight the number and copy it, then paste it into the dialled number box of the Search for Dialled Number report.
Top 25 Long Duration Numbers
This looks at all the numbers dialled for that period, then collates and sorts them in descending order.