The best way might be for the company to outfit its trucks with any old cell phones you can find. Verify this, and in particular what year this went into effect, but as far as I know, any working cell phone manufactured after that can call 911- without regard to any existing contract or plan.
Next possibility adds a little time- not good for an emergency, but still better than no contact possible.
I have a Skype number for my main phone at home, and also installed on my TAB 3 7.0 Sprint. I can't call 911, but I can call a police station, or in my case, the State Police. This would also work with any other phone app for your Tab. If your business has a reasonable range of towns that your drivers travel through, you could prepare a list of local police station numbers and/or add it to the phones memory. As long as your drivers know the town they're in, and the lists/phones are set up, at the time of need this should work quite well.