Hi , i found this method will help you
Reset Network Settings. 99% of your success rate will depend on this. (You can find this in settings - general and to the bottom you'll see a "reset" menu in which you can find this)
Reboot your iPhone
Wait a little over a min.
Remove your SIM card and re-insert it back in
Wait 30 sec.
Turn on Airplane mode and wait 10 sec. (This is important as well)
Turn of Airplane mode, you should see "searching" and should pick up the carrier's signal.[/size]