iPad owners know where the FaceTime app is. So where is it on the iPhone? The answers is: well, there isn't one. But you can, of course, still make FaceTime calls. Here's the explanation.
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iPad (2 or later) and iPod touch (4G or later) owners are well familiar with the FaceTime app.
However, if you also have an iPhone (4 or later), you may one day notice that there is no FaceTime app on the iPhone. Where did it go?
The answer is that there isn't one. Apple has elected to make FaceTime available directly inside the Contacts app. The reasoning may be that the iPhone is a device more oriented towards person-to-person communication, so it makes sense to offer the FaceTime opportunity, on the spot, in the context of looking up contact information for someone.
If you look at an entry in Contacts for an entry, you'll see the FaceTime button at the bottom.
FaceTime is accsessed via Contacts on the iPhone
And you're all set. Just tap it to start the call. Of course, the other party must be using a FaceTime enabled device. (It could be a Mac, running OS X 10.6.6 or later.)
By the way, Apple offers some good advice on using FaceTime in the Knowledge Base article # ht4319. If you're trying to contact someone with FaceTime on an iPhone, try their mobile number first. If you're calling someone with an iPad or iPod touch, use the e-mail address they've registered.
While we're at it, how do you register the email address you want people to use when calling you? That's found in Settings -> FaceTime. It's generally your Apple ID address.
Settings -> FaceTime manages your FaceTime "address"
Note that on an iPhone, your carrier may allow FaceTime over its wireless network. There's a switch at the bottom (circled in red above) to restrict it to Wi-Fi only so you don't burn excessive minutes over 3G/4G/LTE with FaceTime.
Now, you're all set to use FaceTime on your iPhone, even without an explicit app.