You should be registered for My Sprint and provided a phone number associated with their account are able to view unbilled call details online.Only account owners or account authorized profiles can view unbilled call details for all lines on the account.
Use this steps to check unbilled call details
Sign in to My Sprint at www.sprint.com. My Account page is displayed.Scroll to the device section, click on the See All Usage linknext to the phone or device that you want to check. On the Detailed Usage page, click See call log.