Vonage's recently announced V-Phone is a USB memory stick with a difference; it has a built-in voice-over-IP (VoIP) client, and a jack socket into which you plug an accompanying headset to make voice calls.

The concept behind V-Phone is one of those ideas that seems brilliantly simple with hindsight; you should be able to make a VoIP call from just about any Internet-connected PC, if the software to do this were more widespread. But just about every PC has USB ports, so why not put the software on a USB device, and carry it around with you? This way, you don't need to install anything, and you don’t leave a data trail behind you, either.

Perhaps the most surprising thing about V-Phone is that it actually works; my previous experience of Internet phone calls led me to expect a crackly, hissing line, with frequent complete breaks in the sound.

Instead, V-Phone provides perfectly good audio quality. At times it was a little quiet, but the Vonage Talk client provides separate volume controls for both the incoming audio and the mic level, allowing you to adjust the sound to suit both you and the person at the other end of the line.

However, anyone hoping to get cheap calls wherever they go in the world should be aware of two things that can get in the way; privilege levels and firewall configurations.

I found that you need administrator rights on a system in order to run the Vonage Talk client, or even to access the V-Phone's built-in Flash storage. I don't know whether hotels and Internet cafés routinely leave their public-access PCs running with full admin privileges, but locked-down systems will prevent you from using the V-Phone.

Then there are firewalls. The Vonage Talk client needs access to a number of UDP ports to make IP calls, and these may be blocked by a firewall on both the PC and the Internet gateway of any network you are connected to.

I was unable to make a connection to the Vonage server from the network in IT Week's offices, for example. At home, I had to temporarily disable the firewall on my Windows 2000 system in order to make calls.

The V-Phone costs £19.99 in retail, with a subscription to the service costing £7.99. This lets you make unlimited calls, and Vonage also assigns your V-Phone a number so that other people can reach you whenever you have the device connected to a computer.