What should I do about an RIAA phone call?

At 11:55 pm I got a phone call saying that I downloaded 30 songs illegally and am going to be charged $677000 from the phone number 202-775-0101. They said it was the RIAA and they wanted a message to defend myself against prosecution. I didn't know what to do and panicked and just let the message go. The problem is I've never downloaded music, and the only thing I've ever done is convert YouTube videos to MP3s a couple times. I don't know what to do and am very nervous. I legit do not have Napster, Limewirse, Ares, any of that stuff. I don't know what to do and am looking for advice. HELP!