To fix the FaceTime activation message, follow the steps below:

1.) Turn on WiFi by going to Settings > WiFi and connect to your local WiFi network.

2.) Go to Settings > General > About and wait here (about a minute) until you receive a carrier update prompt. After the update, the carrier field should say “xxxx 7.2″, where “xxxx” is your carrier name. If the carrier says “xxx 8.0″ or later, continue to step 3.

3.) Now go to Settings > Phone. Turn off FaceTime and turn off “Show My Caller ID”. Then, turn iPhone 4 off (slide to power off method).

4.) Turn iPhone 4 back on. Go to Settings > Phone. Turn “Show My Caller ID” back on and turn FaceTime back on. Make a call to another iPhone 4.