I spent 6 days and about 8 hours every day trying to figure out how to make Facetime work on T-Mobile with no luck.
Then I heard about an application on Cydia that makes it work, it's MiTime. This application make the iPhone 4 send a SMS international to activated Facetime.
Of course, the application costs $7.99 but only once. It's worth every penny. Trust me, my wife and friends are very happy because we can have a very good video chat.
Here's the instructions:
Step 1
Add this source to Cydia http://cydia.pushfix.info
Install MiTime via Cydia from this source.
Step 2
Turn off FaceTime in Settings->Phone. MiTime will turn it on when appropriate and this will not waste international SMS messages.
Next, run MiTime and and follow it's instructions. Most of you should get to the "Check Compatibility?" screen.
The number you see can be changed if it is wrong for the SIM you have inserted in your phone. Only change it to the correct number for the SIM in your phone! If you need to change anything other than the country code or area code, you will need to contact the creator of MiTime via Twitter.

Step 3
Especially if you are outside the US, check that your phone number is listed in correct international format. For US customers, this is +1 then the 10 digit phone #. For international customers, it should be + then the country code and then the phone #.
Step 4
Tap the "Test" button to check MiTime compatibility with your phone, carrier, and phone plan.

Step 5
Hopefully, you will get to the next screen which shows that you are compatible.

Step 6
It now shows you a code that you can use to purchase

Step 7
Once purchased, close and restart MiTime and your FaceTime® will be activated.


Step 8
The app can be used to fix almost any FaceTime® problem in the future just by running it.

Notes after installation.
A) If you turn off FaceTime® or it gets turned off somehow, don't turn it on in Settings -> Phone. Run the MiTime app and it will fix it and turn it back on.
B) You may need to make a FaceTime call before your phone is reachable by another iPhone 4 via FaceTime (especially after reactivation through A) above). You only need to attempt to make a call because the first call you make may fail, but after that you are "registered".
C) If you ever see the warning message about costs again, tap Cancel and run MiTime to see what is going on.
Once again people, this works. Remember, it cost $7.99 but it's worth it!!!
ENJOY!!!
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