Roaming at Home

My MPx200 Smarphone has been on the fritz for a while now. it misses more calls than it used to and it refuses to sync over USB. This morning my replacement phone (a refurbished one) came in the mail. I popped my SIM card in the phone and turned it on. It synced beautifuly. About 30 munites later I noticed that the phone wasn't listing AT&T wireless as the carrier. Apparently “O2 - UK” has a tower somewhere near my building. After cycling the phones power an couple of times and talking with 4 diffrent people at AT&T It's been determined that I received a bad phone. Hopefully my replacement will be in tommorow but in the mean time I'm a bit curious as to what was going on. Why couldn't my phone (even when I did a manual search from the Telephony settings screen) find any US Networks? and how was it able to find a UK network here in Seattle?

I want an MPx220...