When you answer the person on the other ens stays silent, and will wait until you hang up. If you don't answer they don't leave a message, so that would be the wise thing to do. Just ignore it!
I recently changed my number, and I have since gotten two calls from 718-351-1877 - an automated debt collector calling to confirm that I'm someone by the name of "Perkins" who owes money. The message says: "by continuing to listen, you acknowledge that you are (Perkins)..." I would hate to have to change my number again! I hope it doesn't come to that...