I spend a decent amount of time in the backcountry--and more likely than not I have no reception. In general, if I can't see buildings my phone probably doesn'

I spend a decent amount of time in the backcountry--and more likely than not I have no reception. In general, if I can't see buildings my phone probably doesn't work. (Note that "see" should be interpreted broadly--looking down from a mountain you can rarely resolve individual buildings, but you can see it's developed land.) Major roads will generally have service even if you can't see buildings, but secondary roads rarely do.