Wouldn't work. "911" isn't a real phone number; it's special-cased into the phone network so that if the phone network is overloaded the 911 call will get through and a non-911 call will get disconnected. VoIP services don't have direct access to 911 systems but instead proxy (once they figure out where you're calling from and thus which 911 center to route your call to).
If you tried to DoS 911 services via VoIP, you might make it impossible to get a call through to 911 via that VoIP service, but you wouldn't block landline 911 calls.
I don't really understand what you mean - do you mean that 911 won't accept more than X calls from a single VOIP provider, after they proxy the call? Because the limit is humans answering the calls, isn't it? Is that done on a grid too, so operators from multiple regions can handle overload in one area?
Anyway, I would imagine a more likely attack would be on private PBXes. Only terrorists would want to take down 911, which should be resistant to call floods, but a much larger pool of criminals would seek to disable private phone systems.
Do you think the 911 quotas for all the VoIP systems added together amount to more than the staff for the 911 center, in some locale? How about the number of emergency responders? Even if not, you could still DoS the VoIP service your target was using.
Wouldn't work. "911" isn't a real phone number; it's special-cased into the phone network so that if the phone network is overloaded the 911 call will get through and a non-911 call will get disconnected. VoIP services don't have direct access to 911 systems but instead proxy (once they figure out where you're calling from and thus which 911 center to route your call to).
If you tried to DoS 911 services via VoIP, you might make it impossible to get a call through to 911 via that VoIP service, but you wouldn't block landline 911 calls.