Can I use the T-mobile contract free iPhone 5S with my AT&T number? I am thinking of purchasing the contract-free 5S because my contract with AT&amp;T just expired, and I will only need the phone in the US for a few more months.  Will I be able to use my AT&amp;T sim card (I currently have a 4S) when I receive the T-mobile contract free 5S? Also, I assume I will be able to take it abroad with me and use an international carrier once I move abroad at the end of the year.

Thanks in advance.
Yes, as long as you buy it from Apple's online store.