Don't stop them, it's the opportunity of a life time!
err, I mean in the UK we have something called TPS (Telemarket Preference Service). If a number is registered TPS it's illegal for telemarketing calls to be made to it.
You probably have something very similar in Canada and the U.S, it's just not many people know about services like that because they aren't advertised and companys won't reffer people to it as it's losing buissiness for the industry.
Look into it for your area, i'm sure you will find a reputable phone company providing a block for telemarket calls (In the UK it's "B.T" [British Telecoms]).

...and i've had someone blow whistles down the phone and most are of poor disturbance level. Rape alarms are where it's at.