You could analyze one of your cell phone bills or just make a good faith estimate. As long as your actual cell phone bill doesn't differ from your estimate by much, you should be okay.
Most cell phone services provide a list of all the calls you made on a detailed bill. You can usually find it online. If you highlight the common personal calls and count them, you will probably get a good estimate.
If this is impossible or too difficult, make a good faith estimate.
