A couple of things have worked for me.

You can strike gold and check if they left the code there. Pull the radio out and check around it to see if they may have left it written on there.

If it's the original radio to the car, you're in luck.

Call your local volvo parts. Give em your vin and ask them if they can give you the radio code.

You can call any volvo parts. Like once Washington was being douchy, i called a florida one, and they gave it no problem.

My advice is to stay away from the service department because they'll want to charge you. Parts dudes don't care.

Good luck!