What is that sound?

Yesterday Ryan and one of his friends made plans to get together today. His friend, who(whom? never sure which is right) had my cell number but not the house number, called and left a message on the cell, I told Ryan to use the house phone to call him back. Ryan tried, and came to me holding the phone with a strange look on his face. "Mom it's making a weird noise, it's just doing this beep thing!" (I was on my cell at this point and told him to try again). He did, it was still making that strange sound. I tried, and guess what... apparently my son has no idea what a busy signal sounds like. I tried really hard not to bust out laughing until I walked out of the room.