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You say you tried using the original code for the existing radio so I assume your radio is not the original one? If so, you will have to take it to a Volvo dealership to have them look up the code. They will just need the radio and proof of ownership (some won't even ask for that) and they will be able to look it up with the serial number. There won't be a charge.
Since you have tried the incorrect code 3 times, your radio is now locked. You will need to unlock it before you can enter the correct code. To do this, turn the key to position ONE and let the car sit with the radio on (the display will read "OFF" but it is actually on) for about 2.5 hours. It will then read "CODE" and you can input the correct code the dealership gives you. When you do this, make sure nothing else is on to prevent battery drain (blower fan, defroster, seat heaters, etc).
If you bring the radio to a Volvo dealership, they can look it up without the VIN from the original car.
You will still need to follow the unlock procedure above before you can enter the correct code.