what one does it say?? awaiting activation, restricted network, or no service? All would be a different fix!
what did you use to unlock it? rensn0w then ultrasn0w? did you get any errors when you tried after a restore?
finally, how did your brother get it? Apple store or 2nd hand? it could be that the imei has been reported and its been blocked.
Where are you using it now? I am not aware of US phones imei's being blocked in other countries