How to unlock a network carrier locked to an iphone 4?

Hey guys i just got the iphone4 running iOS 7.0.4 from United States of America and it is locked to AT&T carrier network which i did not know and am residing in Cameroon but now how do i get my phone to work with our local network carriers and that is all i have on me to make calls and msg. Please help me, is there anyhow i could do it for free cause i don't got cash on me.

Ben, there's an option which I know about opening the locking system of a mobile phone, I did help one of my friend, who got his mobile locked in a puk code, so I'm not sure whether this same method will help you solve your problem or not, but I feel that you should try this method, if this works here, call your toll free network number, then select the options they speak in a recorded voice, then select the number keys as they say, I think this will solve your problem.

Hi, I'm Gohar Khan, I wanna share here how to unlock iphone 4 modemfirware(baseband) 04.12.09 or 04.12.05 with ios 7.0.3
* Before 6 Months i brought iphone 4 from uk ebay online store with network orrange after 1 month i go to dubai for my job then i saw my iphone 4 has locked i shocked coz i pay 425$ in ebay and that guy tell me its factory unlocked then i call Orrange UK company they confirm me that my iphone not unlocked they said its locked and now they not offered factory unlocking, i'm very sad coz i use my lovely iphone4 as ipod.
before 2 weeks ago i search in internet for iphone 4 unlocking then i found this great solution call Superior sim Ultra + its such a greatest Unlocking sim below i wrote how to use for those people they had iphone4 and locked and they wanna quick and low cost unlocking. Coz Its Take arround 1 minujte hardly to unlock any iphone 4 in my case i upgrade to ios 7.0 and my iphone 4 have baseband change to 04.12.09.
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