VoIP firm Jaxtr on Tuesday announced the hiring of Taneli Otala, the former chief technology officer at MySQL, as its vice president of engineering, and the startup said it has increased its registered user base to 5 million in 220 countries.

In July, the Menlo Park, California-based startup said it had doubled its user base to 500,000, and in August it announced that it had received $10 million in funding from a group of investors led by August Capital. (see Jaxtr Sees Jump in Users and Jaxtr Takes $10M Collect Call)

VoIP firms generally offer some of their basic services for free and sell higher-value services to some percentage of their user base, so the challenge is rapid customer acquisition and the conversion of high-end users to paid services, IDC analyst Will Stofega said.

"We looked for a long time to find somebody to help us with the growth challenges we currently have, and if anybody can do it, Taneli is the guy," said Konstantin Guericke, CEO of Jaxtr and co-founder of LinkedIn.

The next hurdle for Jaxtr, Mr. Guericke said, is driving revenue and profitability from the 10 percent or so of its growing user base that the company expects will require higher-value services.

"Our plan was to start our paid services by the end of this year; however, because we are focused on keeping up with our growth, we've decided to postpone that until next year," Mr. Guericke said.

Jaxtr's service, which allows people to send voice links to their long-distance or international friends and chat on their landlines, work phones, or cell phones using local numbers, is currently free.

The basic Jaxtr service allows people about 100 minutes of incoming calls per month.

"We are starting to get people running out of their minutes by the 10th or 15th of the month, and we designed the service with a pay service in mind, so we didn't give unlimited free calling away like Skype," Mr. Guericke said.

Mr. Guericke does not see Jaxtr as a Skype rival; rather, his research shows that a high percentage of Jaxtr's market is people who generally make international or long-distance calls using calling cards.

"Our users are comfortable using landlines or mobile phones but not computers when they make phone calls, so they are less likely to be Skype users," he said.