Unlocked Razr - Which carrier to use for prepaid? (T-Mobile, AT&T, services)

[Tampaite]

I bought an unlocked Razr phone.

I would like to use this phone for pre-paid(or pay as you go) plan.

Am I limited to T-mobile and At&T since the Razr phone has a SIM card ? or can I use this for Sprint or MetroPCS services?

Also, how do I know if this phone is a U.S version or an International version?

Thanks for your help!