Existing and Previously Existing Hard Lines would still work, but prolly not Cell Phones.
There is a requirement in FCC law, at least in FL, that previously occupied homes/apts/condos etc that hard hard lines, still be able to call 911, even if no one currently has an existing phone line hooked up.
Theoretically, you could plug a phone into a preexisting phone line and call 911.
For example, my sister got rid of her hard line; uses cell phone only. The lines are still there. She can plug a phone in, and call 911 even if she doesn't pay for/have access to reg phone workings.