Phones like the iPhone have the ability to use most american, european, asian, and FAIK african cell phone bands, for years now. Most high end android phones have similar abilities (and some allow multiple sims).
My iphone has never been unable to communicate on a region's network, and I travel a lot, My (unlocked) iphone has worked on all four continents mentioned. Quite a few places in America, most countries in europe west of Czech (and who'd want to go east of there), South East Asia, and Morocco (OK, not all of Africa).
Of course, maybe el cheapo brand cell phones might differ, but if you are paying $20 for a cell, who cares if you can unlock it and use it again?