At the office, we used to double team 'em...a call would come in, John would answer and the telemarketer for office supplies would ask for the serial number on the copier; John would ask them to hold on while he got it.
I'd give it a full minute and then pick up asking if they'd been helped, and they'd say John was helping them.
I'd tell them John was at the library, and they'd be stunned, saying they thought he was getting the serial number.
At that point I'd break it to the caller that we used the library's equipment.
May their hell be filled with telemarketers all calling each other selling ice water futures.