Because FaceTime is a built-in app for iOS, I see a FaceTime icon on my phone's desktop and we click on it to open it.
Then, after opening the FaceTime, we go directly to the phone address Book to call the contact person, or enter the phone number into the caller. If the other person can use FaceTime then it can be like a phone to get through and the other side can be with us video.
As shown in the following figure the two sides after connecting FaceTime video call, you can chat like QQ to see both sides behind the things oh.
Attention:
With FaceTime you need to log in FaceTime, you need an Apple ID. If you have an iTunes Store account, MobileMe account, or other Apple account, you can use the Apple ID with FaceTime. If you do not have an Apple ID, you can create one when you open FaceTime, if no AppleID recommend you: Apple ID Registration method