While Apple is a global brand, they do manufacture phones for specific areas of the world. The phone they do buy should work in whatever country they will be traveling to, but as far as LTE is concerned, that might be an issue. Every phone can connect to a 3G network if this is the case making the phone usable.
Every phone that is manufactured and shipped is made for a specific region in the world.
I bought my phone in Canada (at an Apple Store), and live in Canada. My phone will work on LTE networks in Canada, the States and Mexico. Outside of that I have no idea, but at least I can get some sort of service even if it's H+ for the Internet (3G on the iPhone). Both my iPhones (6S Plus, 128GB version and an 8 Plus, 256GB version) were bought in Canada at an Apple store in Toronto.
You regulars can correct me if I'm wrong.