Cell Phones Increasingly Used for Prank Calls

All cell phones have the ability to make a 911 call even if the service on the phone is disconnected.  This is a requirement of the Federal Comunications Commission, and while this is an obvious safety measure, it's also being mis-used by some to make prank calls to 911.
Emergency dispatch operators say they're getting an increasing number of prank calls and hang-ups made by children playing with disconnected cellular phones that can still be used to make 911 calls.

"Kids call us and swear at us because they know we can't do anything about it," said Clearfield Dispatch supervisor Wendy Brimhall.

Brimhall said dispatchers are helpless such calls come in because emergency-call-only phones do not show a callback number or any location information.

Although this was a local story, it's not isolated to that one area of the country.  I've heard this same story on NPR previously.  My question is, though: if kids and teens are using these phones for these calls, how are they keeping them charged?