@ All reflash your ROM and check: Settings > Wireless and networks> More... > Mobile network > APN/Accespoints.
If these are empty you need to add them manualy.. For this you need to check the website of your operator..
guys i have flashed a french ftf and now i dont have the gsm only mode !! why ??
Dial *#*#4636#*#* from the keypad.
Go to phone info tab. scroll down.
Check preferred network type. you can change it whatever you want from dropdown list.