IPhone 4, Switching carriers from Verizon to T-mobile?

I'm debating on getting the Verizon iPhone 4, it'll be free but it doesn't have a Sim card slot. How would I change from Verizon to T-Mobile without a Sim card

You can not. No sim car no t-mobile. T-mobile is only compatible with AT&T iPhone 4 Verizon AT&T and Sprint iPhone 4S and 5.

Cant it is only free with a two year contract. Which means you can only use it with verizion otherwise you hav to pay full prics for te phone. And tmobile sucks dont get it

You cant use an iphone with tmobile no matter what cuz its not compatible