I found this little gem over at Freeware Pocket PC. This application will will log all incoming and outgoing calls and publish them as private appointments on your calendar.
This is great for logging when you talked to someone, or you can just got back and edit the new appointment and add notes directly to it.
After you install if you must either launch it manually by browsing to the installed file location, or you may simply reboot your device and it will launch upon the next boot automatically.

Installation is quick and easy and the program works as advertised. Best of all it is free!

So head on over to Freeware Pocket PC to get your copy