Jaxtr on Monday said it is signing up more than 12,000 users per day and that it has doubled its user base to a half million registered users in the past month.

The Menlo Park, California-based VoIP startup also said it added 18 new countries, including India and Russia, to its list of places where people can make international calls from cell phones for the price of local calls.

IDC analyst William Stofega believes that the emergence of new players that focus on blending VoIP into existing apps represents a changing of the guard.

“The legacy models are under tremendous pressure,” Mr. Stofega said. “Big telecom has not been able to innovate thus the search for new models of service delivery in which voice becomes more application less utility.”

Independent VoIP providers have taken some hard lumps. U.S. market leader Vonage has indicated that it could be on the verge of bankruptcy. Also, SunRocket, a well-funded startup, recently shuttered its business. Both Vonage and SunRocket competed with traditional phone companies primarily on price.

Startups such as Jajah are moving toward the application model, and firms launched more recently,   Raketu among them, have blended voice with social networking. (see Jajah Expands VoIP Service)

Jaxtr—which has taken only $1 million in funding from Draper Fisher Jurvetson, Mayfield Fund, and Founders Fund—has a very similar business model to Raketu (see VoIP Startup Has Portal Dreams).

Jaxtr integrates its service into popular destinations such as MySpace, Facebook, LinkedIn, Orkut, Craigslist, eBay, LiveJournal, and Blogger.

“It is clear to me that VoIP is much better suited to the social-networking medium and cell phones, because a lot of the social-networking generation don’t even have landline phones,” said Jaxtr CEO Konstantin Guericke.

This story was originally published on July 23, 2007.