Can I use my US phone in Europe / Europe in US?

The HTC made PocketPC Phone is a dual band phone. Because of the greater proliferation of GSM in Europe, the European versions of the phone (primarily O2) are setup to use two European bands, and not the US. Therefore, a European phone can't be used in the US without modification. The US version of the phones are setup for the US band, and the most popular European band. In this case, the US phone will work worldwide, but may not offer as much service in Europe as the European version. There has unverified discussion about the possibility that the hardware is identical, so that the European phones could be reprogrammed to cover the US. This has not been confirmed and therefore is not adviced.