Incidently BOAC I bought a Nokia last year from Amazon and used an existing SIM card from Vodaphone, could voice and text but every few days I'd get a message saying I should adjust my multimedia settings, which I was unable to do, also couldn't send pictures. This is why I personally am a bit wary of trying to use a very intelligent device on a different SIM card. Having said that I am too busy playing with 'everything else' the HTC provides which is not related to the phone network. Put Skype on it and once you're in range of a WiFi spot who cares if the 'phone' works !
Good luck.