To make it short, anyone that ported his/her number to AT&T from a different carrier for the new iPhone 4 will have issues using Facetime, read the full story to understand the problem and the solution.

Here we go:

On the 24th I upgraded my iPhone at an Apple store.
I also got a new iPhone with a new line and ported the phone number from T-Mobile. (2nd phone)

The Apple store employee that helped was very nice and the new iPhone worked great, including FaceTime.

Once my T-Mobile number was ported FaceTime stopped working.

The iPhone could not receive FaceTime calls but could initiate FaceTime calls.
When I tried the caller ID that appeared on my UPGRADED iPhone were the Other iPhone New Temporary number I got until the port from T-Mobile completed.

So from that I understood that FaceTime servers registered the phone number as the FaceTime user name, but the problem was that the servers registered the temporary number (a number ATT gave us until the port from T-Moblie will complete).

So I started troubleshooting the iPhone, this is what I did:

Restarted the iPhone
Tried using FaceTime with other people.
Reset the network settings
Called AT&T to reActivate the sim card.
Restore the phone from backup.

Nothing worked… So I called AT&T Again – no help.
Called Apple Care / technical support – no help.
Went back to the Apple store- nothing.
Called AT&T again and they put Apple Rep on the line – no help and a very rude response.  (first time from Apple)

The AT&T rep that helped me finally understood the problem from being with me on the phone for hours trying to find a solution and she got AT&T engineer on the phone that fully understood the issue and thanked me for bringing it up.

He had no solution but he gave me his personal phone number to discuss the progress of resolving it.

He said that he will work with Apple engineers until it will be resolved.

He also understood that this is a problem that will welcome any new (porting) AT&T / iPhone user.

In the meantime I kept on trying to fix the problem my self so I tried to:

Reset the iPhone content and settings and reActivate with iTunes. Finally it worked.
But when I restored the phone from backup it stopped working again. So I figured out that theres a file (plist or cookie) on the phone that saves the Facetime user information.

This time I reseted only the phone settings. And when I tried FaceTime again the phone registered the right number.

I know that most Apple reps can not understand the problem or give a solution.

So try just Reset the Settings (only the settings) and it should work.

I tried it on 3 different phones and it did the job.

* Please leave a comment if my solution helped you.

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