WCDMA uses the same system as gsm (uses interchangeable sim cards) but is used on 3g networks running hsdpa because it was found that the old system (tdma) could not cope with the exponential growth of mobile phones and the fact they will consume more of the network's resources. All WCDMA phones are also equipped with old style gsm radio for when there's no 3g coverage. One worry about roaming to north America is that WCDMA uses different frequencies than the rest of the planet but for straightforward phoning and texting and edge you should fine since the 2g bit of the phone is quad band.
Source: cloudy memories from communications technology lessons, please correct me if wrong.
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