The home page of the control panel shows blocked calls on the right
To see the details of a blocked call hit the 'Details' button
Reasons could include not having enough credit available on the account, they could have been blocked due to the call being made from an unrecognized IP or the call may have been been barred due to your International and Premium Rate Calling rules.
Depending on the reason of the call being blocked, you will be presented with an appropriate button to the right of the listed call to take relevant action. This could be a link to top up your account if calls are blocked due to insufficient credit or an allow button to add an exception to your call barring for the number dialed.