Ya a mencionado Dracot que la radio no es solo software, si no también hardware, tal como se explica en el wiki de desire z en
xdadevelopers:
G2 - Band I (2100) and Band IV (1700/2100)
Desire Z - Band I (2100) and Band VIII (900)
NAM Desire Z (Bell) - Band II (1900) and Band V (850)
If you do not have the correct hardware radio bands for where you are using your phone (i.e. for the frequencies used by your carrier), then you will not be able to use 3G at all. So you will be stuck with voice calls and 2G data (EDGE at best).
Mirad esto:
http://www.htcmania.com/showthread.php?t=363476