UMTS and the USAIn the US, AT&T will offer UMTS 1900, meaning that it will operate on the 1900MHz spectrum only; this is due to the fact that the spectrum has already been allocated in the US. UMTS phones that are sold for US use will probably not work on other countries UMTS networks. This is similar to the situation with current GSM networks in the US, which uses different frequencies from the rest of the world. This could change, as the FCC has made moves to free up spectrum in the 2100 MHz band, its obvious global roaming is considered an important issue.
Currently, there is no ability to roam between US and European & Asian UMTS networks; however, roaming between European and Asian UMTS networks is perfectly possible.