If you register a SIP account or create it on Ring.org (that also means registering a SIP account after all), the wizard proposes you to enable STUN. You may want to do it if you are connected under a firewall. Actually, this is not an option of this account in particular, but of the Ring configuration; it will be applied for any account. If you enable this option, you have to choose the STUN server you want to use (e.g.: stunserver.org).