A year ago I had an awful experience with this.
We were on vacation at Big Bend National Park, which is hours away from everything in southwest Texas. When trying to pay for breakfast, our card was denied. I tried to call the card company to tell them that it was OK, but couldn't get through - there was no cell service. Outside the restaurant was a pay phone (remember them?) that I was able to use to call their 800 number.
I learned then that they'd actually flagged my card as stolen, so I could no longer use it at all, and to get it turned back on I needed to receive the code they were sending by SMS and read it back to them. The thing was, we were in a dead cell area, we couldn't get the SMS. And Big Bend is mind-bogglingly huge - 1,252 of square miles of mostly desert (there's a whole mountain ranged entirely contained within the park). As far as I could tell, I didn't have enough gas to drive out of the park to get to cell service to achieve this (the park is so big that it's got its own gas station in the middle, and I'd intended to use this - but without my card, how can I?).
It seemed a perfect trap, there was no way we were going to be able to get out. What eventually saved us was that the hotel manager overheard me shouting at the card people, and came out to give me a map, with the places inside the park that can get SMS text highlighted. Using that I was able to fulfill their requirement.
They never were able to tell me why they flagged the card in the first place. They told me that they advise all card holders to warn them when they plan to go out of state. But I live in Texas, and I was in Texas when the charge triggered. They just shrugged that off.