How to use my iphone with tmobile?

I have the iphone 4 that I used with At&t but now I have the galaxy s2, had to switch last year to tmobile onto a family plan. I really want to use my iphone again, how do I do it and how much will it cost?

Craig'slist (jailbreak)

You have to get it unlocked then all you do is put your SIM card in and it will work

Get if factory unlocked at services like iphoneunlockedcheap.com or others on the web. That way you can use any SIM card from any GSM carrier in your iPhone

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