i bought a xperia u on amazon unlocked, everything it's ok but when i switch to WCDMA i have no signal and all turn off (signal) it only works on GMS (2G).
It sounds like you bought a phone whose 3G frequencies are not compatible with those of the networks in your part of the world. If that's the case, then there's nothing you can do because this is a hardware difference between the different variants of the phone. Unless of course there are any networks where you are that work on this phone's 3G frequencies.
