Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
You can make a FaceTime call from Contacts or by tapping the FaceTime icon while on a voice call. On iPad 2 or later and iPod touch (4th generation), you can make calls using the FaceTime app.
If you encounter issues making or receiving FaceTime calls, try the following:
Verify that FaceTime is enabled in Settings > FaceTime.
If the issue persists or if you see the message "Waiting for Activation", try toggling FaceTime off and then on.
If you don't see the FaceTime setting, ensure that Camera and FaceTime are not disabled in Settings > General > Restrictions.
Verify that the date and time are set correctly:
Tap Settings > General > Date & Time.
Toggle Set Automatically on.
Verify that both parties are connected to a Wi-Fi network and have an active Internet connection.
You cannot make or receive FaceTime calls using a Cellular Data network connection.
FaceTime uses push notifications to send and receive invitations. Verify push functionality using Mail or other applications that use this feature.
Restart the device.