Using an iPhone 6s from USA in Europe
When I traveled to the EU with my iPhone 6s having Verizon service on it, it worked perfectly fine there since it's a world phone.
Although verifying with your carrier would be the ideal thing since carriers could have rules of not accepting phones not branded by them and also to verify for 100% that the LTE bands are correct for their network but as far as operation wise I could confirm that I used my iPhone 6s in the EU back in March.