Using iPhone 6s in Europe
Is it possible to use an iPhone 6s, (US purchased), in Europe?
Question marked as
Depends on what you mean by "use".
An unlocked iPhone will work with sim cards from most if not all GSM cellular service providers in Europe. You may not get full service in all areas depending on band support by the carrier however.
Also if using an American cellular carrier, you should be able to have roaming service there with a local european carrier that has an agreement with your U.S carrier (if there is one) for such service without using a local sim card or having an unlocked iPhone.