I recently upgraded my Lumia 900 to 920 at AT&T, so I want to unlock my 900 so that I can use it in another country. I got the unlock code from AT&T. However, it just doesn't work. When I was asked for an unlock code on Lumia 900, I input the 8 digits code, press Enter, and it said "incorrect PIN". I've called AT&T services tons of times, and it ends up that they asked me to call Nokia support. The IMEI I gave to them is 100% correct, and AT&T stated again and again they've given me the right unlock code. Is there anything special to input the unlcok code on Lumia 900?
Nokia do not issue restriction codes. Restriction codes are a matter for the network who has locked it. The procedure opnce you have the phone is to insert an alternative sim and power up the phone. The phone will ask for a pin and all you have to do is to enter the pin.
However, you only have a finite number of attempts to enter the correct code. If I'm not mistaken, you have 10. The pin will no longer work once all the attempts are used up. The screen requestion the pin will also tell you how many attempts you have left. My guess is someone was playing with your phone and used up all your attempts.