RingFree is a newly launched "application" for the iPhone that enables SIP-based calling from your iPhone. You can call people using your SIP-based IP PBX or other SIP-based provider. You can also make free VoIP calls to FWD, MSN, Yahoo, Google Talk, and anything with a SIP URL.

RingFree was started by Voxilla-founder Marcelo Rodriguez, who whispered to me about this a couple of months ago. Much of the expertise gained from Voxilla is being applied to this new product, and it looks to be a winner.

RingFree is a web-based application. There is no need to "jailbreak" or otherwise modify your iPhone in any way that might void your warranty. Whatever carrier your iPhone is locked to will be happy as well since the call travels over the GSM network to RingFree's servers, where they communicate via SIP to wherever the call ultimately goes.

You have to sign up for the service from your iPhone, which is currently free. Since I don't have an iPhone, and they don't support other device manufacturers yet, I can't try it out. If you have an iPhone, give it a try and let me know what you think!