Telemarketer - subscriptions to technical bulletins. As soon as they launched into their spiel I zoned out so I didn't catch the company name. They obviously had no idea who they were calling. They were just trying to reach someone to sell to, or get information from. I remained mostly silent throughout the call, speaking just enough to keep them talking and wasting as much of their time as possible. Hopefully they won't call back.
An Indian guy asking for my IT Manager but he calls from different numbers and portrays different names and is always with a different company. I either leave him on hold or get as much information from him as possible then tell him that he will never speak to my IT Manager so STOP CALLING!