Contact Facetime availability detection

Default apple Contacts app detects, whether the selected contact is available via Facetime. If it recognizes the person is available, the `call` and `video` buttons are enabled (and vice versa).

What is the API the app uses to determine given contact is Facetime available?

Looks like checking the URL (via `UIApplication.CanOpenUrl`) ability to open returns `true` if Facetime is just installed (regardless of the contact id provided).