What makes entrepreneurs into billionaires? A drive to succeed, a ton of hard work, a good idea, and being in the right place at the right time. Those are the usual answers, and they're all true. But there's one more equally important item: A willingness to boldly approach strangers or near-strangers to try to sell them something seemingly outrageous. And then do it again and again until enough of them buy.

It struck me when reading a recent GOBankingRates piece about the habits of 24 billionaires how many of them had worked in sales, particularly door-to-door sales, the most unforgiving form of the cold call. That's difficult news for a lot of people (myself included) who shudder at the very thought of making sales calls to strangers.

But history doesn't lie. Take a look at this list and consider whether it's worth spending the time and effort to learn to be better at sales calls. For myself, I think the answer is yes.

Warren Buffett sold chewing gum and soda door to door.

He bought them from his grandfather's candy store and then sold them at a small profit. That habit of knocking on strangers' door served the legendary investor well. In 1952-at 22-he decided he might want to invest in GEICO. He took a train from Omaha to Washington, D.C. to visit the company and learn more about it, only to arrive on a Saturday and find the office closed. No matter, he simply knocked on the door till a janitor let him in. Not only did he wind up investing in GEICO, he also became lifelong friends with Lorimer Davidson the VP he met that day.

Bill Gates contacted strangers to sell a product that didn't exist.

In 1975, 19-year-old Bill Gates read an article about the Altair 8800, a new microcomputer. He contacted the computer's manufacturer to say that he and his friends were creating an interface to allow the computer language BASIC to run on the Altair. He didn't let the fact that he didn't have an Altair and could therefore write no code for one slow him down. He mainly wanted to find out if the computer's makers were even interested.

They were, so Gates arranged for a software demo. That left him and Paul Allen a few weeks to a) create an emulator that would let them program for the Altair without actually having one, and b) create the software he'd promised to demo. They made it, and the Altair's maker became Microsoft's first customer.

An impoverished Elon Musk sold his city guides to big media companies.

Years before Tesla and even PayPal, Musk's first company was called Zip2. It provided an Internet city guide for big media companies in 1995 just when those companies were striving to get their online acts together. That's the only explanation for how a 24-year-old Musk was able to get contracts for Zip2′s guides with The New York Times, Los Angeles Times, and Chicago Tribune.

They might have balked had they known just how close to the edge Musk was. Unable to afford an apartment from which to run a home-based business, he leased an office where he slept on a futon and showered at a nearby YMCA. The up side is that, once he was at the Y anyhow, he often also worked out. "I was in better shape than I've ever been," he jokes now.

Phil Knight cold-called a Japanese CEO.

In Japan, where protocol is primary and appointments and sales must be made through the proper management layers, such an act is almost unheard of. But Knight was passing through Kobe Japan on an around-the-world-trip after getting his MBA when he encountered Tiger brand running shoes. He was so impressed with their high quality and low cost that he finagled a meeting with the company's CEO. In that meeting, he secured distribution rights for Tiger shoes in the Western United States. Those shoes, when they arrived, gave Knight the basis for the company that would soon blossom into Nike.

A fired Mark Cuban went after his former employer's customers.

The future Shark Tank star was fired from his first real job at a software retailer for disobeying orders and going to a sales meeting instead of opening the store. Imagine the nerve it took (after he got over his hangover) to start phoning his former employer's customers and ask them to take a chance on a company that consisted of a single 24-year-old working from home, without a computer to run software on.

As you might imagine, the response was what he calls "a lot of let's stay in touch." But two customers bit and one even offered office space. It was enough to give him his start.