IT MAY seem counter-intuitive, but one of the most-used apps on my mobile phone is, in fact, a phone. It is not the phone that comes with the phone (if you get my drift), but a VOIP phone.

In a nutshell, VOIP (Voice over Internet Protocol) uses the internet rather than the phone network to make and receive calls. The calls are routed to the phone network only at the destination point. This turns long-distance calls into local calls, at a fraction of the cost.

VOIP providers all offer pretty much a similar service, but with variations. I use a few VOIP services for different purposes. My main office phone is provided through Vonage.

You will need to continue line rental of your existing line to provide the internet connection to your home or office, but for £8 per month you can make unlimited UK calls, and for just £3 extra you can add unlimited landline calls to Ireland, and unlimited landline and mobile calls to Canada, China, Hong Kong, and Singapore. If you pay £4 per month on top of that, you can add unlimited landline calls to 68 other countries — half of them including mobile numbers — as well as 400 minutes to Nigeria, Bangladesh, and India. Another £4 option provides you with unlimited calls to UK mobile numbers.

But this is all about a home- or office-phone service. What has this got to do with apps? Well, this is the clever bit. Vonage has developed an Extensions app, which acts, as it says, as an extension to your phone line. Calls to your office number are automatically routed to the app, so that you can answer office calls when you are out of the office (or choose to answer on the app instead of the office phone, if you are in the office).

What is more, you can use the app to make calls, too, and, instead of being made on your mobile phone and incurring call charges, calls made through the app are routed through your Vonage account. So, if you’re staying in a hotel with free WiFi, you can make calls, in effect, for free, using the app, if you have a Vonage account.

There are, of course, other types of VOIP account. Many are free to download, and provide free calls to other people using the same app.

Skype is the oldest and possibly the best-known. It offers free voice- and video-calling to other Skype users, and you can buy airtime to make low-cost calls to landlines and mobiles around the world. You can also rent live phone numbers, so that people without Skype can use it to reach you when you are away from a phone but have access to the internet on your laptop or tablet.

When it works well, Skype-to-Skype calls offer better clarity than a standard phone call. Some radio stations have started using it for “down the line” interviews in place of the traditional ISDN line, but the call quality is dependent on decent internet speeds at every step of the process.

Facebook’s Messenger app has developed from a text-based messaging service to a Skype-rival, also offering voice- and video-calling. Its servers don’t seem to be as resilient as Skype, but if you want to converse with a Facebook friend and you both have access to the internet, you can make free calls to each other wherever you are in the world.

RIM’s BBM offers a similar facility. Originally built around the Blackberry Messenger, BBM is also available as an app for iPhone and Android users, and provides a useful back-up to other messaging services.

Finally, Google’s Hangouts app is also available. Like Messenger and BBM, this allows you to make free calls to other Hangouts users, but with the added benefit that you can call most US and Canadian phone numbers for free.