Fring, the maker of a free mobile application for activities like instant messaging and voice over Internet protocol (VoIP) phone calls, has raised a third round of funding.

The most compelling part of the London startup’s offering is its VoIP support. Basically, it means you can make phone calls through your phone’s data plan rather than using cell phone minutes. That, combined with the fact that the Fring app is free, can lead to big savings. Last April, Fring bragged that it was the first company to bring VoIP to the iPhone (although it required users to jailbreak their phones). That’s a bit less exciting now that VoIP leader Skype has come to the iPhone.

Fring says it has millions of monthly users, with half a million more joining every month.

The size of the round was not disclosed, but previous Fring backers North Bridge Venture Partners, Pitango Venture Capital, Veritas Venture Partners, and VenFin Limited all participated. We wrote about Fring’s $12 million second round back in 2007, but now TechCrunch reports it was closer to $10 million, and that prior to the latest funding the company raised a total of $13 million.