Need Unlock Code for Nokia Lumina 520

I have taken my Nokia Lumina 520 to two (2) different cell phone repair places to get them to unlock my Nokia Lumina 520. My carrier is Rogers. Both these places have tried two different unlock sites and have not been able to get a code to unlock this phone. Does anyone have an idea where I can find the code to unlock this phone?

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Nokia lumia 520 AT&T
IMEI: 353045067948693

Need free code please!
Thank a lot !!!