How to unlock nokia, IMEI:352920/02/289670/1
It's not possible to unlock this model manually or by updating the firmware, (it's possible with some models like E51, etc..etc..) but the only way you can unlock for free and the simplest way is to call the service provider, in your cases, At&T and ask them to give you your unlock code. They will ask you your name, your phone's imei number, you phone number, etc..etc.. and will give you the unlock code. But there's one more condition, they'll not simply give you the code, but you have to persuade them wisely, like you want to use two sim cards as you are going somewhere, or you want to use the other sim card or sim cards for your personal personal purposes like business, for to contact your family, etc..etc.. If you tell them that you don't want their network, they'll never give it to you and will ask you to pay if you insists which is not acceptable. But if you ask them that you will continue with their service and will also use another sim card for temporary or for other purposes, they will definitely give you the code.
Many/most service provider lock their phone with their network so that they can get more customers and it's their business policy/trick to attract and lock their their customers and so they don't simply give out the numbers without any applicable condition.
There are several website where you can get the unlock code and you'll be asked to pay for the service, but don't trust them, their services are not 100% successful, the only thing they are doing is to get your unlock code from your service provider and pass it to you, nothing else, so it's better you directly contact your service provider and get the unlock code.
There's another way, but the service isn't available everywhere (in countries like india, we can get it unlocked from any mobile repairing shop where services for nokia mobile is available), so in your case, I don't think it's possible. If so, you can give a try, they will charge you for the service but it's 100% sure that your phone will be unlocked to any network, means free network with the available network bands.
If you need further help, let me know.
Good luck.
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