I do this...

Telemarketer: *Whatever the fuck he's saying.*
Me: H-Have-Have you ever had nnnnasal sex? I-It's when y-you get fffffucked up the nnnnose.

And make sure you say the nasal sex part in a geeky voice.

Once a telemarkter called, and I said "Just a minute..." Then I put the phone next to my speaker and played Scatman by Scatman John. Later, he said "Sir...sir...what was that?" I told him it was Scatman, then I yelled "NAIROBI!" and hung up really fast

Havent heard from him since....