Calling 911 from an unregistered cell phone?

Factual Questions
1

I recently upgraded my phone and donated my old phone to a shelter via my carrier (Sprint).

I was told that carriers can not legally block a 911 call, even from an unregistered phone.

a) Is this true?
b) Is this federal or state law?

2

Looks like it’s anFCC rule. I learned in my first aid class that if you call 911 from an unregistered cell, you have to tell the operator what city/address you are at, and if you are disconnected you have to call them back because they have no way to call you.

3

Thanks.