Iphone 4s : International Use and Switching Carriers

Hello All,

I've looked through a ton of threads and I think i'm more confused than before.
1) I travel international a lot for work but don't want the ridiculous carrier international rates. If I get it unlocked (like the normal Sprint unlock) I should be able to use an international sim on it no problem correct? Is this the same with ATT and Verizon without having to hack it?
2) If I do have to hack using Gevy or whatever method should the iphone work perfectly international?
3) Using Gevy is it possible to switch carriers like Verizon<->Sprint<->Att ?

Thanks a lot!

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