Truphone has launched a mobile phone SIM that allows users to make VoIP calls when travelling in other countries, without being subject to constantly changing roaming charges.

The Truphone Local Anywhere service must be used in conjunction with software installed on handsets, to let users make calls that use GSM in the location being called from, with as much of the call then being routed across the internet using Truphone's servers. Although the company hasn't specified pricing yet, it is thought it will be in line with the company's current price structure, and offer just one flat-rate for the calls.

In addition, the company claims that Truphone Local Anywhere will chop mobile roaming charges - a major complaint for users of conventional mobile networks - by up to 80 percent.

Separately, the company has a service that depends on using Wi-Fi for the first leg of the call, completing it without using GSM at all. Call rates for this internet-only VoIP service currently range from $0.05 (3.5p) to phone landlines while in France, or $0.25 (17.5p) to a mobile.

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"Until now, the only way to avoid the massive cost of mobile roaming is to carry multiple phones and multiple SIM cards, one for each country you frequent," said Truphone CEO Geraldine Wilson. "This means juggling multiple devices and phone numbers to avoid paying the high roaming rates traditionally associated with travel.

"Now these internationalists can enjoy all the benefits of home wherever they may be," she said.

Other benefits of the service include low-cost dialing between countries while roaming, the use of virtual numbers so that users can appear to be 'in' the country in which they are roaming, and cheap calls to other Truphone users using the same service.

Truphone is available as a free download for an expanding of platforms, including Nokia/Symbian, Apple's iPhone, the BlackBerry and, more recently, Google's G1 Android.

A small catch with the new Local Anywhere service is that it isn't scheduled to start until "later this year", so interested users will have to hang on for while yet.

See also: Nokia N97 gets Skype VoIP software