How to activate FaceTime

http://support.apple.com/kb/HT4319

Check Settings>FaceTime>On and Settings>General>Restrictions>FaceTime>ON. On the 4 and 4S FaceTime is accessed through contacts. Go into your Contacts, access the contact you want to FaceTime with and you will see a tab at the bottom of their contact card. You can also access FaceTime by going into your Phone app, select dialling keypad, dial number, press call and an option for FaceTime on the keypad will appear. It should be noted that FaceTime has been removed from iPhones sold in some Middle Eastern countries at the express wish of the respective Governments.

But my phone keep on an error occured during activation. what should i do?

what is that error? did you check out the settings as said above?

Sign in to FaceTime on your device using a valid Apple ID. After you validate the email address for use with FaceTime, other FaceTime users can connect to you using that email address. If you encounter issues registering or using an Apple ID account:

• Ensure that you have an active Internet connection.
• Verify that you are using a valid Apple ID at myinfo.apple.com. You can also create an Apple ID or reset your password from this website.
• Check that your email address is verified in Settings > FaceTime.

  If Settings > FaceTime shows your email status as "verifying," follow the instructions in the verification email that was sent to complete the process.

• Toggle FaceTime off and on in Settings > FaceTime.

iPhone automatically registers your phone number for use with FaceTime during activation. If you encounter issues activating or using FaceTime with your phone number: