Having dedicated code path for 911 is literally the problem. It makes it legally almost impossible to test in many countries (since in many countries you're not allowed to dial 911 if you don't have an emergency)
But most places I've lived in the US had a non-emergency police number, and non-emergency 911 calls were very discouraged. If you have a good reason to make them for network or device testing, you'd just need to schedule a time to make a test call, stay on the line and tell the operator, etc.
Device development would be well served by using a tower simulator in a Faraday cage, as a sibling suggested. But a responsible developer would do a few tests on live networks during release acceptance testing.