What code do I put in to put a T-Mobile sim card into a At&t phone?

HI i think the nokia mobile is asking for network unlock code to use your t-mobile sim in it this is because the phone is already locked with AT&T due to this it wont accept your t-mobile sim until you unlock it .You can purchase network unlock code from online providers like SuperUnlockCodes.com for reliable cost .You first check that whether the code is available for your mobile model .If yes then pay for it and unlock the phone then freely use it with any GSM sim ...