While T-Mobile has long included even slow international data in its plans, AT&T and Verizon largely haven't. For most plans, adding this functionality will cost extra. The good news is that both have gotten a lot better on roaming rates that, depending on your trip, it could make sense to do this rather than going through the process of unlocking, finding a local SIM card and playing that game (we'll get to this more in a moment). 

In the past, roaming was charged on a per-megabyte or -gigabyte used system, which could quickly lead to bills worth hundreds of dollars (or more). Today, both Verizon and AT&T allow you to roam with unlimited talk, text and high-speed data for $10 per day on many of their respective unlimited plans.

If you're traveling with a family plan, AT&T will only charge that $10 to one of the lines with other members being billed at $5 per day -- and it'll only bill you for 10 days, per line per billing cycle. So if you have one line with AT&T and are traveling for two weeks, you could only end up paying $100 instead of $140 for international roaming (though if your AT&T bill resets during that window, you'd be on the hook for those days since the clock starts each billing cycle).

Verizon doesn't cap the number of days it will charge you, but it does offer a couple of different options. One is its TravelPass perk, something that is offered as part of its latest unlimited plans. With this, you can get three days of international roaming for $10 per month. If you don't use those days, you could roll them over (and bank up to 36 of them). You can similarly add it for the month you're traveling, save a little there and then get rid of the perk for all the other months you wouldn't be using it. 

Verizon also offers a $100 monthly plan option that lets you get unlimited texts and data as well as 250 minutes for traditional calling (as opposed to using a messaging app like WhatsApp or FaceTime). Of the data, 20GB will be at "high speed" and the rest at "3G speeds." Note that with this plan you will need to remove it manually or else risk it being a recurring charge on your bill every month.

T-Mobile similarly offers the option to buy high-speed data "international passes." It has a few options here, including a 15GB of high-speed data option (to be used "up to 30 days") running $50 for the month.