There's a number thrown around about a sales person having to make something like 7 to 12 calls to connect, and most giving up after 3. I'm not sure of the veracity of the statistics, but anecdotally I can tell you it sounds about right (on all counts).
It does amaze me when professionals who have invited me to contact them will not return my calls. I've gone the distance sometimes, and I've given up from time to time. Both scenarios are disheartening, frankly.
And that's why we should return calls when people try to sell us stuff. It's just good practice to treat others as we would like to be treated, and further, it's crazy to think anyone will (or should) treat us with respect if we can't afford that courtesy ourselves.