What about the electrical charge carried by a live body?
I very much doubt this equipment is designed to detect electrical charge. My understanding of the technology is that it uses a form of image sensing and looks at the fingerprint pattern. Responsibility2nd's point about a pulse being required is within the capability of an image sensor, so I believe that requirement is plausible, and indeed, an obviously good idea to prevent people from faking it.
Bingo, it requires a live person who has a bit of charge going to the phone. Dead fingers won’t cut it. It’s why you can’t use a piece of wood or metal to write on the phone, you have to use a special utensil that transfers that charge or whatever. Chi?