When your on wifi you dial the person you want to FaceTime then press FaceTime. To my understanding that's what I read you don't need an special codes or anything just the phone number of the person. You can Begin a phone call then by pressing FaceTime your video calling. Once you start FaceTime the other person will be asked to accept.
Here is where you can get more info on FaceTime.
Have you tried calling your partner first? If you haven't called your Facetime partner at least once (starting Facetime from the in-call options), the Facetime feature button in the contact info and at the top of the iMessage list (when texting your partner), is unavailable until you initiate Facetime during a phone call first (Facetiming someone over an iPod Touch or iPad does not require a phone call first), you can also try Facetiming using a registered email instead if the phone number
There are very few requirements for Facetime to work; however both devices must be properly configured:
-Must be iPhone 4 or later
-Check your Settings - Facetime - "ON"
-Must have Wi-Fi connection
-Must be a 'single, point-to-point' call (no conference calling)