Problems with installing SIM card onto IPhone 4?

So I got an unlocked and non-jailbroken IPhone 4 today, and I tried cutting down the SIM card from my old phone and sticking it into my IPhone. No dice. It says "No SIM" at the top right. I have TMobile as a service provider by the way.
The SIM card's a bit loose, but not too loose (By this, I mean a little bit smaller than the adapter). What do I need to do to get the SIM card to work? Do I need to call customer support and ask request a microSIM to myself all the trouble? Or what?

Two things.

First you can get a free micro sim card at any T mobile. Trust me I work there. We usually charge $10 but if your cool. We can give it for free. But even if they charge u. Its worth it. Your phone will perform better.

Second. Not all unlocked iphone 4s work on t mobiles network. Only ATT's iphone 4 works. The Sprint and Verizon version of it don't have T mobiles 1900 band. Which is the band that unlocked phones run on T mobiles network. So even if unlocked it won't connect to the network.

I suggest trading in the iphone. Most T mobile stores offer in store trade in. And get yourself an i phone 5 in payments. If its a 4s youll walk out with an i phone 5 with little to no money.

Hope this helps.

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