Iphone use on cellular south

Not sure if i posted in the right place so ill post here as well....
Ok so ive got my ulocked iphone working on a cellular south network using the sim card from my blackberry that works on gsm networks...my question is:  If im using the gsm capability of cellular south yet im in the US, am i still within the realm of my unlimited data plan. Better yet, if i do this am i charged roaming?? thanks for any help