How to Make Video Calls On Your iPhone/iPad
To make a video call via your Apple device both the caller and the receiver need to have the Facetime application. You can connect a Facetime call using either Wi-Fi or the Optus Network.
To make a Facetime call over the Optus Network. Both the caller and recipient must have an iPhone 4 or iPad 2 (or higher) that's running iOS 5 (or higher). It's important to note that Facetime calls via the Optus Mobile Network incur video calling rates.
Need assistance using your iOS device?
Animated instructions/troubleshooting are available for both iPad & iPhone at Mobile Device Help
If you need more information about Facetime, visit the Apple Support Website.