My Inc.com blogging colleague Jeff Haden recently asked me to share my favorite productivity secret with him. I figured that as long as I was sharing it with him, I might as well share it with everyone else.

My secret productivity weapon (and time management tool) is a little gadget which I bought on Amazon for $14.99. It comes with a cable and some Velcro; total installation time is less than 60 seconds.

The gadget is a switch that disconnects my landline. If the toggle at the bottom is facing away from my phone, the phone is disconnected; if the toggle is facing towards my phone, the phone is connected. It's that simple.

I use this gadget to keep my phone inactive except when I want to 1) make an outgoing call, or 2) take a previous-scheduled call. The gadget ensures I'm never interrupted by phone calls when I'm focused on the task at hand.

Yes, I could probably do the same thing using the buttons on my phone, fussing around with the voicemail settings, or simply unplugging it. But why bother? The gadget makes disconnecting and reconnecting my phone as simple as operating a light switch.

(Just as important, if you work in an office, you can install this gadget yourself and use it to disable your phone without flagging to "big brother" that you're not accepting calls when you want to get some work done.)

This ability to instantly disconnect and reconnect my phone has been a huge productivity boost. Here's why:

According to one study, "unnecessary interruptions consume about 28 percent of the knowledge worker's day." Chief among those interruptions are unwelcome phone calls, like from recruiters, salespeople, and coworkers who can't be bothered to email.

What's more, interruptions are a huge source of workplace stress and phone calls are especially stressful because, unlike email, they command your attention. Even if you let the call ring through to voice mail, the noise (or the thought that this call might actually be important) can and will break your concentration.

Now, one could argue that disconnecting your phone means that I'll miss calls from customers, prospects or coworkers who really DO need my attention. That's absolutely true and it's not a bug, it's a feature.

While I value my clients and colleagues and I work hard for their success, I need them to realize that my time is also valuable... too valuable to be put at risk at their convenience.

If people want to contact me, they can use email, which I check pretty often... at MY convenience.

Which leads me to the REAL secret of making this gadget work: train callers to use email rather than leave voicemails.

Unlike email (which you can read and review in seconds) voicemails are time-consuming and often lead to voicemail tag. Therefore, I train callers to use email by explaining in my outgoing voicemail message that I no longer listen to messages:

I installed the low-tech gadget (and stopped listening to voicemails) at the beginning of 2016. As of this writing, nobody who's really wanted and needed to get in touch with me has failed to do so.

In fact, I've been able to significantly expanded my client base because I can get more done without the constant interruption. I estimate that the raw productivity gain has added at least $28,000 to my yearly income.

And that's not even counting my lowered stress level. It's a wonderful and powerful feeling to know that my phone isn't going to suddenly jangle when I'm deep in thought or writing something important.

So as far as productivity goes, that was by far the smartest $15 I ever spent.