Can i buy any iPhone 4S from amazon and put in my chip from any carrier?

I'm thinking on buying my uncle an iphone 4s for her birthday but i'm not sure if shell be able to use it with her carrier, which i don't know what it is. Will she be able to use it or do i have to buy a carrier specific iphone from amazon?

Do you know who his carrier is? I would figure that out and look on amazon for iPhones under his carrier. Hope this helps!