If it were me, I start messin with them. Next time it happens, breathe real heavy and ask them in psycho kind of voice "wanna hear my c*ck?" Then rub the phone on your shirt or something. Then say "Oh, ya like that don't ya? Ya want more?" then fart in the phone (or at least so they can hear it real good). Then come back with "Oops, I let one go when you get me all excited." Then repeat until they hang up.
Screw it, what are they going to do? They called you, so I seriously doubt that they could get you in trouble, it wasn't a crank call you made, they did it.
I used to do that alot, sometimes different. There was a time when I played Medal of Honor online alot, and the freakin telemarketers were pestering the hell out of me. I got to the point where when the phone rang, I'd check the ID, and if it said anything other than someone I knew, I'd hit the talk button and toss the phone in front of the speaker. They'd sit there and listed to gunfire and grenades and al the other sound effects while I played. they started getting smart with the computers and I had to start saying "Hello" to get their system to transfer the call to a real person. They don't call anymore now, I kind of miss it in a sick twisted way. The bastards don't like me anymore.