I started the phone but FaceTime is not installed.

Correct, but your phone is from a different region. The iPhone 6 will most likely have been for a different region. If you check the model number at: Settings - General - About - Model = ??? Compare the two phones, you will see that The letters at the end are different.

You have a phone from a region where FaceTime is banned or restricted. It is not possible to use FaceTime on that model, while using in Angola.

Where was the phone purchased?

Where are you using it?

What has changed?

Has FaceTime ever worked on that phone?

Other iPhones not from the Middle East can use FaceTime.

No.

That's correct.

It doesn't matter if you currently use your iPhone in a country where FaceTime is not blocked.

You bought it in a country where it is blocked (being Saudi Arabia), so even when you leave that country where you originally bought it, the service is still blocked. There is no way to un-block that feature.