My wife, son, and I recently spent 5 weeks on vacation in Nicaragua. My wife and I brought our iPhones with us, but obviously turned off cellular data and only used them with wifi. Anyway, fast forward to our return from the trip and my wife gets her phone bill- roaming charges to the tune of $33! What gives right? We had our data turned off the entire trip! Anyway, turns out, they were charging her roaming charges for people calling her back home and leaving a message! As you can imagine, we were not impressed. This was apparently a mistake on our phone company's part and this regularly happens when people visit certain countries (for some reason this is a problem when you travel to certain countries and not others). They would never say anything to you though, you have to catch the mistake yourself.
tl/dr: Read phone bill, found $33 "mistake."