Nokia :: 2680 Unlocked Handset Bought From Independent Provider Would Work On Any Network

I purchased an unlocked Nokia 2680 handset in the UK this month to replace a broken handset for my pay-as-you-go O2 sim card. I was told by the representative that the phone was tri-band and would work in the United States, where I live presently. I am emailing from the US where the phone does not work. It does not roam with my UK network (o2), nor will my AT&T sim card work in the phone (sim card registration failed).Any ideas? I thought an unlocked handset bought from an independent provider would work on any network??