It's actually a federal law that cell phone companies have to pass-through 911 calls in the US even if service is not active. That is one reason your phone will say "SOS" when it can't reach your particular cell provider -- when "SOS" appears, it means it can reach another provider that will let your dial 911.
This, by the way, is not true with Satellite services. For example, if you have a Garmin Inreach, and discontinue service, you cannot reach emergency services.
That's why some women's shelters will accept old phones without service. The phones can still be used in an emergency.