Cell phone calls?

I have to keep records of cell phone calls. I have been getting some unwanted phone calls for last two weeks and it is very irritating to entertain such calls. Guys, I have to keep records of my all cell phone calls, so that I can trace the number or person further, if he talks in abusing language. But I have no idea about such process. Guys, help me to keep records. I am using Verizon network service with three years contract deal. It is very important for me. I would be very happy to get solution with the help of your experience.