Assuming the Sony Ericsson phones have both the 850 and 1900 mhz frequencies, T-Mobile or At&t would be your best bet in the US.
I have been looking at the Sony Ericsson cell phones and and the Nokia cell phones and was wondering on what network carrier can I use those phones in the U.S.A? The thing is though I don't live in a major city in the U.S. I live in a kind of small town in northern Michigan. I was wondering if my location plays a factor in it. Any info would be appreciated.