If you took a cell phone that only accepts 9V or 12V and changed it to think it was permitted to accept 20V, you could easily fry a phone. Charger will happily

If you took a cell phone that only accepts 9V or 12V and changed it to think it was permitted to accept 20V, you could easily fry a phone. Charger will happily supply it if you trick the phone side. This would require malware on the phone itself, though. Possibly something quite low level.