MrsEB has the right idea. I can tell you based on my last three trips (two to Europe and one to Costa Rica) what doesn't work: trying to stay within your carrier's data allowance on the int'l plan you sign up for...at least on Verizon. Inevitably, I went over (twice on one trip) and got an immediate $25 overage notice.
The problem is that if you are not on Airplane mode, your phone apps are pulling data from the Internet as you are out and about and not on wifi and you go over the limits. Also, googlemaps are very handy to use when finding your way around, but you'll have the same problem. Whatsapp is a great app for free calls to others who have it, but it doesn't work in rural areas very well in my experience.