You mean 911 from a UPS powered VOIP connection? Or landline? Reason I say this, my telco even gives dial tone for people on dry loop, for 911 & line ID purposes.
Cell could be considered backup unless it was a disaster scenario (rare) Like if tower gets knocked down or overloaded. Either way probably screwed regardless haha.
Just read story of people in an area hit by Katrina may not ever get copper put back. No landline/DSL since. Terrible.
Sorry it's off topic. I'm out.
Authorized Service Provider (Contractor) for MTS Allstream. Any views expressed are my own.