how do i install facetime in uae?

FaceTime isn’t available in Saudi Arabia or the United Arab Emirates. An iPhone originating from those countries will only have it if on a small number of carrier networks, all of which are in other Middle Eastern countries.

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You can't. Facetime is banned in the UAE. There's no way to get it on any carrier in UAE.

Wireless carrier support and features for iPhone in the Middle East - Apple Support