“Half the money I spend on advertising is wasted; the trouble is, I don’t know which half.”

This phrase – attributed to John Wanamaker, considered one of the pioneers of early marketing – is one that has haunted marketing for decades. But a recent marketing campaign I managed using a simple marketing hack has shown EXACTLY which marketing tactics work.

The campaign was for a retailer who wanted to measure where enquiries came from so that they could then make informed decisions about where to invest in further marketing and advertising activity.

The retailer was bombarded by advertising sales people for local newspapers, websites, magazines and even radio stations, who all had very compelling reasons why the lion’s share of the client’s meagre budget should be spent with them.

But how do you measure their success? The answer is virtual telephone numbers.

Virtual or ghost numbers are telephone number that aren’t directly associated with a specific telephone line, so they can be assigned anywhere and automatically forwarded to any other phone line.

For this campaign, a series of virtual numbers were bought for every conceivable element of marketing activity. For example, individual numbers were bought and used on websites, vans, magazine adverts, leaflets and flyers. More than a dozen of these ghost numbers were activated, and all redirected back to the client’s main phone system.

Over the course of a three-month period, I was able to identify exactly where every enquiry had come from, and never once did the client have to ask that old chestnut “where did you hear about us?” That’s because they already knew the answer. The client and I were able to log on to the website of the virtual telephone numbers company and see in real time the usage of each number.

And the best part is that these numbers are jaw-droppingly cheap to use. For the sake of a few quid a month, the client was able to refine their marketing activity and increase their investment in activities that were giving them the best results.

Ghost numbers are an absolute must for businesses so that they don’t have to waste a penny of advertising spend – unlike the unfortunate John Wanamaker.