Call Detail Records contain both inbound, outbound, and internal calls. Depending on what tool you are using to export CDRs, and from what you are pulling CDRs (CUCM or CUBE for instance) you may only see a subset.
Check your export for proper parameters. Also, you can access the Call Detail Record Analysis and Reporting Tool CAR Administration Guide - Chapter: CDR Search (You may need to start the CAR Web Service to fire up the tool.)
Maren