Considering that none of the services that can be dispatched by 911 are legally required to help you in most North American jurisdictions (i.e. if you call 911 and the police don't respond until they finish eating their box of donuts, they're not criminally or civilly liable), having working 911 services really doesn't guarantee you anything. Most security monitoring companies have contracts that are completely worthless and guarantee nothing as well.
If you're depending on 911 for life safety and property protection, I'd recommend revising that plan to include a dog and/or gun. 
- Pete