I gave my first iPhone (a 3GS) to a friend who needed a phone because the 3GS he bought had a bad cellular antenna.
This was about two years ago. It was of coursed locked to AT&T so he went to their page to unlock it and got denied. Several phone calls later he had to ask me what the original phone number was. I had no freaking clue because I had changed the number to a Google Voice number in the Settings app.
So, a text to my sister, who had bought this phone new, was made and of course she couldn't freaking remember because this stupid AT&T family plan she's on with her boyfriend (that they consistently try to get other family and friends to join) has been disconnected and reconnected so many damn times their numbers aren't consistent. Further, it was the number she had in Anchorage and she'd since moved back to Seattle at this point.
After some semi-hostile back and forth sparring with her I finally managed to get a legit phone number out of her and my friend was able to unlock the phone.
All of this for a friggin' iPhone 3GS. Great phone, but a reminder to me of why AT&T is called the DeathStar.