Go to Settings, and tap FaceTime to open its options. Tap the slider to turn on FaceTime, and select Use Your Apple ID for FaceTime. Make sure your phone number has a check mark beside it. You can also choose the option to add an email address.
Enter your Apple ID and password, then select Sign In. FaceTime displays your phone number and all email addresses associated with your Apple ID. Note: it only shows your phone number if you are setting up your phone for the first time. Select Next to continue.
Select the email addresses and phone numbers you want people to use to contact you for FaceTime calling. Next, scroll down to Caller ID. Choose the email address or phone number you want to appear during an outbound FaceTime call.
Find an entry in your contact list, and select FaceTime. You can also use Siri to activate FaceTime or tap the FaceTime button during a call.