You must have 1.1.3 or higher firmware installed on your iPhone to install this patch.
Make sure you have added iClarified as an installer source. You can find instructions on how to do this here.
Press to launch Installer from your SpringBoard.
Press to select the Install tab at the bottom of the screen.
Press to select iClarified from the list of Categories.
Press to select Call History Patch from the list of Packages.
Press the Install button at the top right of the screen.
Press the large red Install button that appears.
You will be notified that you must be on the 1.1.3 or newer firmware. Press the large OK button.
A notification will appear telling you to reboot to enable the patch. Press the large OK button.
You will be returned to the Categories menu once installation is complete. Press the Home button to return to your SpringBoard.
Hold down the Sleep/Wake button until the Power slider appears. Move the slider to the right to power off your iPhone. Then press the Sleep/Wake button to turn the iPhone back on. You should now notice an increase in the amount of entries your Call History will hold!