A telemarketer probably can't handle it if you don't follow the script. Keep a copy of Shakespeare next to the phone for those occasions.
"Hello, is that Mr Smith?"
"Now is the winter of our discontent"
"... Hello, is that Mr Smith?"
"A rose by any other name would smell as sweet"
(long pause)
"I'll call back another time."
Mokalus of Borg
PS - I have not tried this one myself.
PPS - I think it has potential, though.