Apple seems to have deliberately imposed restriction for access to call history in its SDK design. The restriction maybe imposed for privacy proposes and for protecting such data from being misused by questionable iPhone applications. However, access to the call history data is possible on the computer in which the iPhone data is backup onto.  The call history can be accessed directly on the iPhone from a Mac or a PC using the DiskAid utility. The call history is physically located under the root folder on the iPhone.  The root folder is normally restricted.  So in order to access to this folder, the iPhone needs to be a jailbroken device. An iPhone can be jailbroken by following the instructions for Windows or Mac. Note that a jailbroken iPhone reverts back to normal when the firmware is upgraded using iTunes.

Another way to access to the call history by application running on the iPhone is to access to its call history database.  The iPhone call history data is stored in a SQLite database.  The database schema for the call history database can be found here.  There are some suggestions on how to access to the database using the SQLite C/C++ API. Note that the database cannot be accessed using standard SDK or API, but through some indirect methods on jailbroken iPhone suggested in here.