I had one of those recently, let them go for as long as they wanted. Then they asked for a credit card number, so I googled fake card number generator and gave them one. At this stage he was getting pretty excited as he thought he'd obviously caught someone, for the next 5 min he struggled with my fake cc number, getting me to read it back. Finally he twigged came back angry and said my credit card number was BS - to which I replied "yeah just like your scam mate". Well he lost the plot big time, all the time I was laughing at him. Felt good.