I have Verizon, not ATT, but I'd follow this rule of thumb if you want to keep the budget down, even if buying the international day pass. If the ATT international day pass works like like Verizon's, you don't have to pay for any day you don't use it. BUT, not just the first call but the first byte of cellular data that you use activates it for the day, so you have to remember to turn off your cellular data because many apps use data in the background even when you aren't using them. You can still use the apps, and wifi calling, etc, but use them on wifi. And of course if you find you need to use an app over cellular while you are out and about with no wifi, like Google maps if you need a little help with navigation out on the road, its still available to you - just turn that app back on for cellular data, and off again when you are done. A little settings management goes a long way on this one. You can also reset your phone's cellular data usage tracker when you arrive if you want to keep tabs.

Yup, got burned once when too many pictures were being shared on WhatsApp, and didn't buy a plan for my wife's phone because I thought we didn't both need it. Oops, thought it was on wifi, but nope, was over cellular. That was a few hundred $ worth of data... doh!