Internet phone service Truphone on Tuesday introduced a plan that offers 40 million BlackBerry users cheap international calling rates to 33 countries.

The service, in test mode, demonstrates the challenging opportunity Internet voice companies such as Truphone face in the mobile market.

“We are looking at developing a software core that is useful across all the various platforms, but today the nature of the mobile environment is about lots of platforms and lots of different devices,” Truphone CEO Geraldine Wilson said.

VoIP, or voice over Internet Protocol, has sent domestic and international rates for landline calls plummeting. But high-rate international calls on mobile phones have so far remained  mostly under carrier control. 

Unlike the landline market where carrier networks and phones are standardized, the mobile market is fragmented. That means there are multiple carriers, multiple mobile operating systems, and hundreds of different mobile phone models, making it harder to gain entry. 

Reluctant to offer their business on a platter, mobile carriers and handset makers share a tight bond making them slow to introduce a technology that could eat their revenue from international calls. VoIP providers have had no other choice but to tread gingerly through the mobile market.

What VoIP providers have done is create software or simple calling procedures to attract mobile phone users to their services. Yet the software must address all of the various barriers, including phones, carriers, and mobile operating systems.

Both Truphone and Jajah have registered their VoIP software in Apple’s App Store, and Truphone has software for Nokia phones. London-based Truphone is now tackling BlackBerry users and it plans to develop software for both Windows Mobile and Android.

Truphone Anywhere for BlackBerry requires users to download the software, which gives them the option of using either their carrier service or Truphone’s VoIP service to make international calls.

If the caller chooses to use Truphone for an international call, the on-board software redirects the call from the mobile service network to the Internet,  where the international part of the call can be made for as little as $0.06 per minute.