You will have to flash a cdma rom from here to your phone. There are quite a few good 2.5 roms here. I would hardspl to make sure it's unlocked. You can do that
here. I would then check out the roms section and pick a rom you want. You can do that
here. If the sim is unlocked all you have to do is put the sim in and you should get voice automatically. To get data to work I would search xda.