Planned to buy iphone 6s

Wireless carrier support and features for iPhone in the Middle East - Apple Support - https://support.apple.com/HT204042 -

"FaceTime may be unavailable if your device originates from this region and becomes unlocked or is used in another country.

Get help if you can‘t make or receive FaceTime calls - https://support.apple.com/en-us/HT204168 - "FaceTime isn't available or might not appear on devices purchased in Saudi Arabia, Pakistan, and United Arab Emirates, including Dubai.""

No. It is blocked at the device firm ware level. The restriction is on devices purchased in the those countries, not your actual location when you attempt to use it.

No. If you buy the phone in UAE Facetime will not work on it anywhere. There is no way to install FaceTime on a phone sold in certain countries where it is banned, regardless of where you are.