U.S. wireless roaming is free on all nationwide AT&T plans. While no additional fees are incurred during AT&T roaming, these minutes are treated like your regular AT&T minutes.
For example, a U.S. roaming minute at 3 p.m. on Tuesday would count as a regular anytime minute whereas a roaming minute on Saturday would count as a free night-and-weekend minute.
International roaming minutes, on the other hand, are billed as separate minutes of use. These can be pricey with AT&T and can go all the way up to $4.99 per minute. These fees by country can be found here.
With AT&T’s GSM technology, AT&T says: “Your phone assumes the characteristics of the wireless network in each country you visit. If you are traveling in Italy, your phone will work like a local Italian wireless device.”
Read more roaming policies by carrier here.