Cold calling was always a difficult way to get sales leads. Consumers hated being called out of the blue and often expressed their anger with profanity or (if the caller was lucky) hanging up without comment.

Calling businesses wasn't much easier. While there was less profanity directed at the caller, reaching a decision-maker meant dealing with gatekeepers whose job was to put cold callers on infinite hold.

Then came voice mail, which meant that fewer calls went through to an actual person. To bypass voice mail, cold callers began using auto-dialers which call multiple numbers until a human being answers, at which point the salesperson is put on the line.

Auto-dialers, however, always have a slight lag, making it obvious to the person being called that they're being auto-dialed. Most people (consumers and businesspeople alike) simply hang up.

To make matters worse (for the cold callers), an increasing number of consumers and even businesses are dumping their land-lines and just using mobile phones. Since it's illegal (at least in the U.S.) to auto-dial cell phones, salespeople who want to cold call must dial directly, a time-consuming process.

Even then, making a connection is iffy and the best a cold caller can expect is to end up in voice mail. That was bad enough before, but not leaving voice mail is rapidly becoming an exercise in futility.

According to the VOIP provider Vonage, each year a larger percentage of voice mails are never retrieved. Millennials, in particular, don't like voice mail and prefer to use texting and email. Voice mail has become so unpopular that some companies are shutting it down. Coca-Cola, for example, no longer has voice mail at its corporate headquarters.

Voice mail is becoming like FAX--a technology that's still in use in many workplaces but increasingly seen as more bother than it's worth.

There's a bigger picture here, though, that is truly the nail in coffin for cold calling.

The entire concept of cold calling is dependent on the idea that normal phone behavior is to answer a ringing phone and have a spontaneous conversation. As originally conceived, voice mail was supposed to be a backup plan to that, providing the person who'd been called the opportunity to call back the original caller, at which point the spontaneous conversation would take place.

What's different today is that people--consumers and decision-makers alike-won't take calls from people they don't know. (Even then only when it's convenient.) Phone conversations are quickly becoming a "by appointment only" activity. People use email, text and calendar programs to schedule a mutually agreed-upon time to speak.

There's no place for cold calling in that model.

This is not to say that there aren't companies attempting to develop sales leads through cold calling. However, they're not getting much of anywhere because the only people who still use land lines, listen to voice mail and answer the phone directly are senior citizens and low-level people at "buggy whip" companies.