Kyocera’s KX21, or better known as the Switchback, is a spanking new device that features a full ... "does this work with cingular sim cards

If you get device unlocked then yes

but not untill then

no the switchback is only on virgin mobile usa phones i have that phone and it dose not use sim cards at all so there is not way to put on in a cong phone u can always use virginmobile its a very good network