Someone can't just upgrade his plan without him knowing unless he's not the primary account holder. Are you sure he didn't go over in his data or minutes (a lot)? How was he unaware that the bill was so high?

Also, getting reception or not has nothing to do with paying a bill. AT&T & Sprint use two different technologies (GSM & CDMA) so coverage is going to vary depending on where you are location.

My friend got his iPhone 4 from Sprint. In my apartment, he gets no signal, neither phone nor internet. Yet, I use AT&T and get a strong signal. Worse yet, Sprint shut-off his phone without warning. He panicked and went to a local Sprint store and was told to call Sprint as there's nothing they could do to help. After being put on-hold for a half hour using my phone, they told him that he has an unpaid bill which he never knew about as he recently moved. They told him he must pay a huge bill (over $500) at once or a collection agency will be notified. He was also told that somebody upgraded his plan to an expensive one, as his bill was way too large. Now, he's stuck with a useless phone and they refuse to allow for payment plans. Is this the same Sprint that shows satisfied customers with great service? Glad I stayed with AT&T !