The Wireless Communications and Public Safety Act of 1999 requires any cell phone be able to call 911 on any network it can find the signal of, even if it has no service available on that network so that you can use deactivated phones or make 911 calls to local emergency services in areas that might not have official 911 service.

There is a difference between a lack of signal and a lack of service. Without signal your phone can't find a network so nothing could work. Without service your phone might be able to communicate with a network but might not be activated, or it might be another carrier's network where you don't have a roaming contract, etc. If the network is there, you can call 911 even if the phone is locked or doesn't have a contract. But if there's no network you're up a creek without a paddle.