mabagal wrote:Tommy, GSM phones from Taiwan will work with AT&T and T-Mobile. There's absolutely no reason why one cannot bring one from Taiwan and get it to work.
It's not that simple. In the USA, GSM broadcasts at 850 and 1900 MHz, while in most of Asia (including Taiwan) it's 900 and 1800 MHz. An unlocked quad-band phone will cover the whole world, but a dual-band phone bought in Taiwan will not work in the USA (or vice-versa). The least expensive cell phones (like mine) are dual-band. There are tri-band phones (not so common these days), which are missing one of the lower bands (850 or 900 MHz). The lower bands are used primarily in rural areas, so a tri-band phone bought in Taiwan might work fine in a US city but not in the countryside. Anyway, you need a quad-band phone - anything less, and you will like encounter problems if you bring it from Taiwan to the USA.
Furthermore, some GSM cell phone companies in the USA require proprietary phones. In particular, Net10 and TracFone will NOT work with any other SIM-card GSM phone - you must use their phones. Doesn't matter if it's quad-band or not, they have burned some kind of key into the phone's memory chips that is needed to connect to their network. Fortunately, Net10 and Tracfone will sell you a pretty cheap phone along with the SIM card. As mabagal indicated, AT&T and T-Mobile SIM cards will work with any unlocked GSM phone, but again, quad-band only if you buy the phone in Taiwan.