There are techniques and services to provide for availability on 800 number routing. You would still be subject to the selected carrier's ability to route your calls out of the local area. Otoh, the car could fallback to 911 if the commercial routing didn't provide positive confirmation.
That said, I don't know if they do fallback, and I don't know if there's any public reporting of availability incidents of these types of service centers like there is for 911 failures.
I don't know how to handicap the wager that CarPlay's center is better than 911's, but I tend to doubt it.