I believe that the pre-paid cell phone minutes expire if not used by a certain date. Also, not sure if the pre-paid phones dial 9-1-1 the same as conventional ones. (Most cells will dial 9-1-1 just by pressing and holding the 9) but it is possible that a pre-paid goes through a intermediate source. (I believe Vonage VOIP does something like that!)
Might want to check those things out, but other than that, it seems like a good idea.