Calls that you make and receive will be charged at different rates. To view a detailed list of call tariffs hover over Configuration and click on Billing. You will be able to check call tariffs for both inbound and outbound calls when you click on See your full tariff in the Billing Overview tab.
Alternatively you can also view individual charges for calls by viewing the Calls report. Hover over the Analytics icon and select Calls. You can filter and generate a tailored report by the call cost column to display calls with the highest charge at the top of the table.