More or less.

I used an unlocked T-Mobile branded phone on both the US Cingular
network and the Norwegian NetCom network and the basic functions of
making and receiving calls and text messages worked fine.

But, the phone had some T-Mobile specific features built into the
firmware involving Web access and instant messaging that occupied
space on the menus and home screen that obviously didn't work.

There are probably features of the Cingular and NetCom pre-paid
networks that (if I'd known about them) I'd have found either
difficult or impossible to use.