this is from their web page
United States Military Gets Free Calling to the US with Gmail
If you have a valid .mil address and would like to make free calls to the US from another country, you must:
- Add a valid .mil email address to your Google account – you can find instructions on how to do so here. If you don’t have a Google Account, create one.
- Install the voice and video plugin.
Free calls for troops and domestic calls will be free for at least the rest of 2011.