They most likely have automated systems to call ranges of phone numbers. Or the random person call them believing that the scammer is actually a legitimate service. If you Google “my computer has virus”, you will find plenty of fake “Microsoft support “ numbers to call.

They only get all the info when they “verify” the info and the victim willingly provides the info. They create a convincing story whether a risk that they claim to be able to mitigate if the victim cooperates, or they fake a prize that requires a certain action to obtain.