To call your friend using FaceTime, you need his phone number or email address,using either phone number or email is determined by the device you are calling:
When calling an iPhone 4, use the phone number of the person you are calling, when calling an iPad 2, iPod touch, or FaceTime for Mac user, use the email address designated for FaceTime of the person you are calling.
On iPad 2, iPod touch, and FaceTime for Mac, you need to add a person as a contact using the FaceTime or Contacts app before you can call them.
Face Time call can be make in two ways from your iPad 2 (Face Time), place a Face Time call using your contacts list.
Contacts, tap the Face Time icon to place a Face Time call to the contact you are viewing.
To start a video call with your best friend, locate your best friend name in your favourites list on your iPad and tap the Face Time button.Instantly an invitation pops up on your friend’s iPad, iPhone, ipod touch or Mac demanding if your friend wants to join you. When your friend accepts your invitation, the Face Time call begins.
Right in front of you, almost life size your friend face pops up on the big iPad display.
With Face Time, you can take part in video calls with another iPhone 4, iPad 2, iPod touch (4th generation), or a Face Time for Mac user over a Wi-Fi network connection.