Here's an interesting idea. Millions of people make VoIP phone calls over the Internet from their PCs, or using special handsets, so why not do the same thing with a humble mobile phone?

There are now several applications that attempt this feat using Wi-Fi connectivity, though it has tended to be a clunky experience or, alternatively, work well enough to rouse the ire of mobile operators which have blocked calls out of fear that the technology could undermine their monstrous call charges.

They are right to be alarmed because there is now at least one take on the idea that works pretty well, and it's called Truphone. When we reviewed the beta of the free application back in October 2006 we were intrigued by it, but the latest version, 4.0, has come on a long way since those early days.

The Truphone concept is pretty simple. Wondering around a city (that's in the UK, North America and most of Europe plus some other locations), calls on a handset running the app can be made in three ways.

Calling via Wi-Fi

You can use a mobile operator and pay the sometimes high call charges, especially if you are roaming or calling from one country to another. That's probably a 'free' call if you have bundled minutes, but will be sky-high if you dare roam from your home country. Step up Truphone. You could also make the same call for nothing using an available Wi-Fi connection as long as the person on the other end is also using Truphone on their phone. If they aren't using Truphone, then you can still make the same call over the Wi-Fi for a much lower charge on GSM, around 3p a minute to a landline.

Worried about finding a Wi-Fi access point that doesn't charge? This will be a problem in some cases, though Truphone does offer free roaming with one of the UK's biggest Wi-Fi networks, The Cloud.

When out of Wi-Fi range, a second Truphone 'Anywhere' mode can route the call using the GMS network, but not entirely via the operator's network, instead using Truphone's as much as possible. This will also save you money in some circumstances (you still pay local connection charges) but it will probably be a good option if making international calls.

Call back mode

There's also an Anywhere 'call-back' mode for roaming users, whereby the server calls the user to set up the call after having been prompted by an SMS. It's slower than making a normal mobile call, and the charging is a mite tricky to work out, but it might be worth it for customers paying high local tariffs in some cases.

So whatever else you can say about the technology that underpins Truphone, it is hardly a bad thing to have as an option, the one heavy qualification being that you must use one from a defined range of Nokia/Symbian dual-mode handsets that can manage both GSM and Wi-Fi. We tested it on the business-oriented Nokia E65.

It's also worth pointing out that while you don't change numbers to use Truphone, users in the UK and USA are given a second 'Truphone number' to use in tandem with the one from the mobile company. Customers then make calls from this number when using the Truphone app and must also receive calls on this number in order to benefit from lower call charges. If someone phones on the 'regular' GMS subscriber number, roaming charges will still apply, so contacts need to know about this number.

The app is easy to install, from which point onwards it's a matter of adding credit to the truphone account, and setting out to find a Wi-Fi access point (AP). Connecting is as simple as handshaking the desired AP from a list offered by the Truphone app, entering a security key if required. Beware one issue. Some private access points are set up to accept only specified MAC hardware addresses, so that will need to be entered in the router config; public access points rarely use this security feature. Helpfully, the Truphone app has a "what's my MAC?" feature to help out.

Truphone installs as a Symbian app, complete with icon, loading itself when the phone is booted. Assuming you're connected to a Wi-Fi access point, all calls are routed through the service by default, although it can be configured to prioritise the mobile network if that is preferred. Truphone calls flash on-screen with the [number]@truphone so you know when it is being used, and there is an audible sound while the number is ringing.