Buy a whistle. The next time they call, if you won't follow other suggestions including the 'do not call' warning, say politely 'Here it comes, CUN-T!!!', then blow the ever-lovin' fuc-k out of your whistle directly into the phone. A maniacal laff prior to you slamming the phone down is recommended.
Be sure to say cun-t, no other word will do - be sure to emphasize that word and blow your whistle as soon as you finish the word.