Q: I can't find FaceTime on my iPhone 6s immediately I insert a nigeria's sim card..but when I insert the one I'm in egypt the FaceTime reappears again

Your iPhone originated from a Middle Eastern country which currently has or previously had restrictions on FaceTime; the Egyptian SIM card is overriding the lock, but no SIM from outside that region will. You won’t be able to get FaceTime on that iPhone in Nigeria.

(144277)

iPhone's originating in Egypt among other countries, may lose access to Facetime if unlocked or are used in another country, according to the footnotes in the following page:

Wireless carrier support and features for iPhone in Africa - Apple Support

       5. FaceTime might be unavailable if your device originates from Egypt and becomes unlocked or is used in another country.