You can use any unlocked tri-band GSM phone as a phone on any GSM networks, but many of them use different bands for 3G. In the US, the only other GSM provider

You can use any unlocked tri-band GSM phone as a phone on any GSM networks, but many of them use different bands for 3G. In the US, the only other GSM provider is T-Mobile, which doesn't share AT&T's bands.