I kind of enjoy some messing with the spam calls.  When I'm in the mood it's a source of entertainment.  The goal is to confuse them.  Whatever pops into my head at the moment.  Often times I pick up the phone and don't say anything.  If the line gets disconnected, I guess it's a computer and took a call before me. 
Sometimes I just start off with, "What ever you're selling I'll take a dozen.  When can you get them here?  What color will they be?  Can you overnight them?  The last shipment was busted when I open the box.  They quickly hang up.
 
One of my favorites is to say. "Hello ....(city of choice) police dept.  Homicide--sergeant Hall speaking.  You have a homicide to report?  Is this an emergency?  Do I need to call 911 for you?  Is someone bleeding?  Did you shoot them?    I just keep asking questions, speaking in a firm, fast tone.  They just hang up. 
I've messed with the folks going to fix my student loan so bad one guy called me a M-Fer and hung up!