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i'm thinking it comes built in the phone's slot and it starts working once you finish itunes activation
 
You don't insert a sim card into the iPhone.

Have a read of this review.

But the iPhone has a major drawback: the cellphone network it uses. It only works with AT&T (formerly Cingular), won't come in models that use Verizon or Sprint and can't use the digital cards (called SIM cards) that would allow it to run on T-Mobile's network. So, the phone can be a poor choice unless you are in areas where AT&T's coverage is good. It does work overseas, but only via an AT&T roaming plan.

Actually, I'm not sure if the means "it doesn't use a sim card (fullstop)" or "it can't use a sim card that works on the T-mobile network".
 
The phone likely comes with a SIM like most new GSM cell phones do...Not sure though if it will be in the tray already or not. Phones generally still have the SIM mounted to the shipping card.

WINNAR!

I'm more interested if I'll be able to take my existing sim card and put it in there, or if I have to use the one in the box (it used to be you had to use the one in the box).
 
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