Trust me, once you disconnect your service you will STILL be able to make 911 calls no matter what.
I was looking in FCC website and couldn't find clearly specified ruling, BUT there has been requirement that emergency line to be available at all the times. So, it vaguely for everybody, BUT what I understand "At all the times" meaning that no matter if service is available or not, the emergency communication is to remain in effect at all the times. So, 911 is one of emergency communication system known in America, so therefore it shall be available at all the times.
Probably because of Patriot act, FCC wanted to ensure that Emergency calls can be made from anywhere with minimal time waste. So, speaking if you can't afford phone line, which is fine by FCC and FCC is concerned if you saw some of terrorist doing something in works, FCC wanted to make sure you CAN call 911 right away, rather than looking elsewhere which can lose valuable time and let Terrorist get away. I think this makes alot sense right now.
