A friend has come up with this tactic for cold callers.
"Can I speak to Mr/Mrs X, please"
"Who is calling"
"Steve Bargain from Draft and Leak Windows"
"Hang on, I go and get him/her."
Put the phone down and put the kettle on. When it has boiled pick up the phone and say "He/she is just coming" and put the phone down again. Make a cup of tea, drink it and then put the phone back on the hook.
It clogs up the phone for a while, but if we all do it, they won't be able to afford to make cold calls, the phone bill will be bigger than their profits.