Cell phones, like elephants, never forget. Or hardly ever. Your phone keeps history records of all incoming and outgoing calls, and even has a record of missed calls that were never answered.
You can find your cell phone history on the phone itself, usually by accessing Call Records (or something similar) in your phone's main menu. AT&T, Verizon, Sprint, Cricket and most other mobile phone service providers also list call records online on your billing account. Log into your phone account and click on View Full Bill rather than just the summary bill, to see a list of your calls. Some sites even offer a download function so you can store the list permanently on your PC.
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