Until my internet connection is stable enough to stay up for years at a time I will not trust my line to 911 and other services replaced. We had Vonage and its just fine, works like a charm, until as usual the internet connection goes down. This seems to happen about once ever month where im at. The phone since I was old enough to use it has never been off.
24/7/365
There is never a maintenance window with your phone even a planned one.