IPhone4s no service/searching

My brother recently got the new iphone 5s and gave me his old Iphone 4s (which was working perfectly)
I got a new sim from my network and activated it with them and also unlocked the phone from my brothers previous network.
However I have put my new sim into the iphone and it keeps showing "searching" and then "no service"
I have contacted my service provider, they tell me the sim is activated (which it is as it works in my galaxy ace) and the phone is clearly unlocked
What can I do to get service running normally? Any help would be great!

This what happened to me what I do Is try ring some one for 5 sec then hang up it should come back