Sony Ericsson Z600

the only way for att or cingular to carry this phone would be if SE made one that worked for both GSM 850 and 1900 Mhz. T-mobile could probably carry the phone as is, but that's up to them.

As far as I know, all the the carriers in north america have their phones "sim-locked" by the manufacturers. This means that you can only use a cingular sim card in a phone that was made for cingular, same thing with att and tmobile. for example, trying to put a tmobile sim card in a phone that was made for cingular and is "sim-locked" will generate errors and the phone will not work. I have heard that it is possible to take the "sim-lock" off of phones, but I dont know how it is done. If you had a phone that was "unlocked" you could purchase a pre-paid sim card in asia (I'm pretty sure that these are popular in asia) and just put it in your phone and it should work, granted the phone you have works on the GSM frequency used in that area.

Hope that helps you