Why doesnt your Volvo radio work after you enter the correct code And how can i get it to work?

each time i want to play the car sterio of 1997 Volvo S70 GLT It will ask of code. It will ask of a code.How do i put this code. the serial number is VO8170 V9179463B. You need to go to your Volvo dealership and they will do it for free. If you call them they will give you a code and tell you to keep your radio on for two hours than put that code in there but that does not work it is better to just go to the dealership they will do it for free and than they will give you the code in case it happens again.