It may sound far-fetched, but you can now make mobile phone calls to almost any country at any time absolutely free of charge. To prove the point, last Friday we took an HP smartphone, loaded it up with Voip software (more of that later) and went to a cafe in central London where I chatted over a mug of tea to Fabiana Jacomini in São Paulo, Brazil.

The call took place during peak business hours, we talked for at least 10 minutes across almost 6,000 miles, and neither of us paid a penny. We could have gone on but I had more free calls to make to friends in New York, Sydney and Stockholm.

The technology that made this possible was produced by Skype, one of a number of companies now offering phone calls over the internet, known as Voip, which stands for voice over internet protocol (see panel). The calls are free because there is no need for the operating company to make the connection, as the conversation is relayed over the internet direct from one user to another.

Until very recently Voip was only available if you were willing to make calls while seated by your computer, which had to be hooked up to a broadband connection and either a headset or an internet-phone handset. But thanks to the growth of wireless internet access, or Wi-Fi, a new generation of internet phones is making it possible to use Voip more like a mobile phone. And, if you’re calling from one of the free public-access Wi-Fi hotspots — I called Jacomini from one near Oxford Street — you won’t even have to pay for your broadband connection.

Rather than downloading Skype’s software onto your computer, you can now buy one of the new Wi-Fi Skype handsets (although they work only in Wi-Fi-enabled homes or offices) or, like me, download the software to a smartphone with a Windows Mobile 5.0 operating system for a more mobile experience. You can then make calls for free to all other Skype users.

“We are now making Skype portable,” claims Alex Kazim, the company’s president. “The idea is that in the future you will be able to take your Skype phone with you and use it in one of the growing number of Wi-Fi hotspots. And Skype calls don’t need mobile phone masts.”

Mobile phone and landline operators are watching developments closely. But Katrin Ruud, a telecoms market analyst for Gartner Research, claims they’re not as concerned as they ought to be. “Telecom companies are not known for their agility, and this puts them in a challenging position,” says Ruud. “They see Skype as small, but it’s successful, and with all the other companies in this area it soon adds up. Mobile providers took customers from landline operators and now internet call providers are doing the same for mobiles. ”

Skype promises all Skype-to-Skype calls “are free and always will be”. The software is also free and can be downloaded from www.skype.com. The company makes its money (it anticipates $195m revenue this year) from charges for connecting calls to non-Skype mobiles and landlines (starting from about 1.2p a minute).

Tesco launched a rival internet phone service in January, which offers free calls between all Tesco internet phone users, and BT offers Voip calls with BT Broadband and has plans to expand its internet call services. Other competitors include Google Talk, Vonage and Jajah (which uses the internet to connect people using their conventional landline or mobile phone via its website), but Skype has led the way in making internet calls accessible and taking them beyond the realms of technophiles.

Launched in 2003, it was bought by eBay last year in a deal worth up to $4.1 billion. Take-up has doubled in the past 12 months and it claims there are now 136m registered Skype users worldwide, of which 5m are in the UK.

But it’s not time to throw away your conventional mobile phone just yet, because there are still big drawbacks. If you want to make a Skype-to-Skype call the person at the other end must be connected to a broadband-enabled computer or be in a Wi-Fi zone. Public Wi-Fi hotspots, which allow free connection to the internet, though growing in number (Bristol, Sheffield and Norwich city centres have recently adopted them, as well as parts of London), are still rare and often patchy, so at the moment you are largely restricted to home and office use. The new dedicated Wi-Fi Skype phones made by the likes of Netgear, Philips, Logitech and Belkin don’t even have the software to work in a public Wi-Fi zone. And the security agencies have raised concerns, saying internet calls make it easier for callers to hide their identity and location.

It took me several attempts before I got through to Jacomini, a public relations executive whose clients include Skype, and the split-second delay before her words reached my ears made for a slightly stilted conversation. I made that New York call on my landline.

HOW IT WORKS

Voip? What’s that? It’s an acronym that refers to phone calls over the internet What’s the point of it?

Cost. Calls between two Voip users are often free. Modest fees apply if landlines or mobiles are involved

Must I be tied to a PC? There are adaptors to convert a cordless digital (Dect) telephone into a Voip one and phones for calling on a home Wi-Fi network

Do these work at free Wi-Fi hotspots? Generally no. Some smartphones with Voip software can

Are calls to other Voip users always free? It depends. Most Voip providers offer free calls to customers on the same service. Inter-service Voip calls are sometimes free

So what are the snags? Voip call quality is variable. Free Wi-Fi coverage remains patchy, but www.free-hotspot.com offers a handy list. Despite the flexibility and savings, it’s unwise to rely on Voip for critical calls — any computer can crash. That said, it’s an exciting future