Hello! At the moment I am working in Europe, but going back to live in US. It's so does happen that next month I am travelling to US for work, and I want to buy a USED iPhone 5S 32GB. My iPhone 4 from AT&T works just fine in Europe, but this far I wasn't able to get clear answers about iPhone 5S: -I have 'heard' iPhone 5S from AT&T will work in Europe, has anyone tried this? -Does anyone know of a phone provider which iPhone 5S also works in Europe? -If I bough iPhone 5S previously from AT&T- do I need to unlock it? -If I bough iPhone 5S from any provider- do I need to unlock it? -Would US iPhone 5S work on 4G network? Thanks much for your help!