The 850 MHz band is preferred as it has better signal penetration as compared to the 1900 MHz frequencies. T-Mobile is primarily 1900 MHz; Cingular is primarily 850MHz. AT&T Wireless had some of both bands. Cingular had roaming agreements with both companies, since Cingular bought AT&T Wireless; they only have a roaming agreement with T-Mobile now. Having a phone that supports 850 MHz will typically allow better coverage. Some rural areas do not have any 1900 MHz coverage, so your phone wouldn’t work there.

If I were you, I would wait a few more months to get a new phone. Nokia will have some quad bands phones coming out. The biggest factor in coverage is the phone.

As for how it’s going to choose another band. In the use the two bands are Cellular (800 MHz) and PCS (1900 MHz). A control channel is used; if you phone doesn’t see the control channel it will try another band. In the case of a Tri-band phone from Europe, it only has one band it could possibly see a control channel on. If it’s a tri-band for the North American continent, it can access two potential bands. It will try the lower frequency first and then the higher ones.