I want to get an iphone from t-mobile but under metro service.how do i do it?

I want to try the t-mobile/att service combined with metro pcs service and get the iphone (any one;preferably the 4s or 5) and i need help on how to do so.

Buy the phone for full price, then get MetroPCS service.

iPhone 5S with no contract:
16GB - $649
32GB - $749
64GB - $849

But Apple doesn't sell the 5s unlocked. So do you just buy the T-Mobile one? Do you have to unlock it to work on Metro?