NonprayingMantis: If it's AT&T then it is possibly locked to their network.
ETA: looks like it's GSM only, not WCDMA, so suprised voice is working at all.
If the phone is locked you will need to get it unlocked, but best to check if it will work on XT before that. They have a IMEI checker online
http://www.telecom.co.nz/mobile/ournetwork/phonecompatibility
which will give you an indication, although not always 100% as relies on the underlying dataset being correct which I think is supplied by the manufacturers and sometimes errors can creep in.