How to unlock an at&t Iphone 4s for tmobile without the original sim?

My dads friend gave him an Iphone 4s running IOS and he already had a Galaxy so he gave it to me. He restored the Iphone to its factory settings on the phone and gave it to me. Its at the activation screen and i trimmed my sim card down to an microsim and it says invalid sim when i'm trying to even get into the Iphone. I have been trying to find a reliable source to unlock it for Tmobile but i can't seem find one. I tried evasi0n but i don't have an at&t sim card for the iphone so it can't be jailbroken. Someone please help me

Added (1). its running IOS 7*

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Evasi0n only jailbreaks phones, not unlocks them.

If the phone is no longer on contract at AT&T you can request that they unlock it. Though this would be better handled by the previous AT&T owner.

Your dad can contact At&t and ask them to unlock it. Another option is to use a trustworthy factory unlocking service like Keys2iPhone.com evasi0n is a jailbreak - it is not an unlock per-se. A factory unlock is always your best bet so try to obtain it. A jailbreak will only work for sometime and it may be a messy process to get your iPhone to work on an unsupported (locked-out) network. Besides, most jailbreaks won't allow you to use some features that your carrier provides and you will need to keep up with future jailbreaks if you need to update your iOS later.