Voice and messaging service provider Vonage is launching a deal offering unlimited international calling for $24.99 a month. But it may not do enough to challenge rival services such as those provided by Skype.

Like Skype, Vonage offers VOIP services which route calls through the internet rather than through phone networks. It’s new World Mobile service is available for the BlackBerry, iPhone and iPod touch. The latter will need a microphone/earphone adaptor and of course only work via Wi-fi, while the first two can work through the data services on cellphone networks.

The key selling point for the firm is that it does not require the user to buy any international calling card or type in special prefixes or pin codes. Instead they simply launch a Vonage app and then make the call. The service might be particularly popular with customers who have friends or family overseas. However, it’s unlikely to take much business away from the calling card market as those products are often aimed at people on lower incomes who are unlikely to have a BlackBerry or iPhone contract.

It’s worth noting that while placing the call, users will rack up domestic minutes with their main provider (unless using Wi-fi) and thus may have to pay extra depending on their service plan.

At first glance, there doesn’t seem any reason why people would use the Vonage service rather than Skype, which charges $12.95 for unlimited international calling. Vonage is, however, offering a $10 discount to existing domestic customers which makes the price a bit more competitive.

Vonage also covers 60 countries, 20 more than Skype. Among the extra countries covered by the Vonage deal are Brazil, India, Mexico, and several Caribbean countries. It also covers all of Russia, whereas the Skype deal is limited to Moscow and St Petersburg.