S.C.
I'm not a guru, but I'll try to answer :)
If Metro uses Sprint's towers then you should be able to use a Sprint phone. I won't swear to it though.
We used to have Centennial, then AT&T bought them. I can still use my old Centennial phone. I still have my Centennial SIM card and it works great in my AT&T phone. The best thing to do is to talk to a representative before you spend money on a phone.