I bought iPhone 6s in Dubai and it's unlocked for europe, but i haven't got FaceTime. How can i download it?

It's not available for download, so you can't.

On the device, open Settings, General, and then About. If the model number has AB or AE just before the slash, it won't have FaceTime anywhere outside the Middle East; iPhones originating from the United Arab Emirates fall into this category. Otherwise, check the device’s restrictions.

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