Jingle Networks, a provider of free telephone directory services, has raised $13 million in a third round of funding, according to VentureWire.

The service, called “1-800-FREE-411″, is pretty simple, and it’s completely automated: You dial a number, listen to an ad and then get the information you need. The Boston-based company has signed up around 150,000 advertisers.

When we covered Jingle back in 2006, the startup had just raised $30 million to build out its network — reaching a “critical mass” of between 10 million and 20 million monthly callers was key to attracting advertisers, chief executive George Garrick said. It looks like Jingle’s efforts were successful; it’s gone from around 13.5 million monthly calls (450,000 daily) to 20 million.

Investors include Goldman Sachs & Co., Hearst Corp., IDG Ventures, Liberty Associated Partners and Comcast Interactive Capital, and Jingle’s total funding to date is $70 million.

This is a crowded market, with lots of competitors offering similar free services, and Jingle competing against big players like Google, Microsoft and At&T. (We also hear that V-Enable has a big announcement coming tomorrow.) Google, for example, is better placed to deliver targeted ads, which is key to making ad-supported services pay off. If Jingle wants to take on companies of that scale, it will have to be innovative.

On the other hand, the startup must be doing something right already, because it reached profitability last month. I’m trying to reach Jingle’s investors to find out more about why the company has done so well, and how it will stay competitive in the future.