How can I unlock carrier after factory reset?

Can you give me the model number of the phone? If it's a at&t phone you can go online and they will unlock as long as the account is paid for and the phone is paid for. You just need to put in the imei number your name and email address. Certain models of phones won't work with certain carriers. If you have a model that is a GSM model it will only work on at&t and tmobile and their subsidiaries like cricket etc. It won't work on sprint and Verizon because they use a different system called cdma. Ok now is it gets weird. Sprint and tmobile will not unlock their phones unless you were the original owner on the account. I already let you know how to unlock at&t and Verizon unlocks the phone after it's connected. I would check and make sure it's not locked and Verizon will unlock it as long as everything is paid for. Ok here it gets fun at&t and tmobile phones will work on each other's network and there subsidiaries like cricket and metro pcs. They will also work on straight talk and family mobile. Verizon will work on those previous networks and total wireless. Sprint will work on all of them. Now this is after they are unlocked. Now days they have made the phones that will work on all networks as long as they are unlocked. I personally don't understand why sprint and tmobile won't unlock the phones if the account is paid for and the phone is paid for and it's not black listed and stolen but you should take that up with the FCC. You can call Verizon and ask for technical support and they can tell you who the carrier was and if it's stolen or black listed and if it has a carrier lock on it (carrier lock is if the account or phone is not paid for they lock the phone until it's paid for) but Verizon is very helpful to find out this information. And if you can give me the model number I can give you an idea of who the carrier was and you can go from there. Verizon phone number 888-483-4937.