The general theory that your mobile phone (cell phone) knows you better than anyone else based on hearing all of your phone calls, knowing all of your texts, facebook updates, emails, GPS locations, etc. and is therefore one of your best friends. After all, there is no person or device that you interact with more than your phone.
Dave and Hal are total cell mates. That guy takes his phone with him with everywhere he goes - even the different rooms in his apartment!