What happens if there's an emergency while driving with no cell reception?

I’ve been thinking about this possibility lately. There are a lot of dead zones where I live. The weather isn’t great and even when it is, plenty of deer like to jump out in front of me while I’m driving. So far, I’ve avoided any accidents, flat tires, etc. But what happens if I end up needing to call for help in a dead zone? Are there any services that would always work whether your cell phone gets service or not (such as OnStar)?