I camp and hike and such in some very rural, mountainous areas pretty regularly, cell service is often very spotty in those places. I know one place off the top of my head that I've been to a few times that I'd have to drive a few minutes away to get a signal.

Off the top of my head, one way around this might be to pair the alcohol sensor with a cellular connection that can call 911, if the car has a signal then the alcohol sensor is required, if it doesn't then the sensor is deactivated. You could also probably have better cellular equipment built into a car than a phone since you don't have to make compromises to keep everything pocket-sized, so you could potentially have more of a signal in more places by calling from your car than you would with your phone.