Just sharing the link here, since I was googling for far too long to dig it out.
How To Use STUN In Applications
It's surprising this is found as a part of the manual for Maemo, the fantastic OS that moved in the right direction, but was practically abandoned by Nokia.
NOTE: This is apparently a guide for the outdated libjingle 0.3.0. I can't find any guide for the new version of libjingle.
If you are looking for an iPhone application to make calls to Google Talk users (and all others who use Jingle extension for XMPP, or XEP-0166), check out Talkonaut. App is currently free on the app store, but I wouldn't be surprised if that changed; in fact, I'd love it if the developers made extra money by doing so. I'd give back the free version and give a few bucks for the paid version. It's worth it.
It even contains Jingle-to-SIP conversion; that is, you can use the XMPP connection to establish a SIP connection. SIP is a "more traditional" VOIP protocol, while Jingle is what Google Talk uses.
They also have partnered with many SIP-to-PSTN providers (where PSTN is regular telephony, such as landlines and cellphone), enabling you to make GTalk-to-phone calls for apparently very low prices. At the moment I have no need for such a service, but it might be a good way to pay back for the free app. I'm not getting paid to say all this — as a developer, I can appreciate the numerous hours that went into this project.