GOOGLE TALK subscribers can now connect with Truphone selected (normally Nokia E or N) wi-fi mobile handsets anywhere in the world.  When you are "ON-NET" with your Truphone, you show up as "ONLINE" through Google Talk, which means you have a free voice call between your Truphone handset and another person using a mic with Google Talk. It is totally free since both Google Talk and Truphone happen through the standard XMPP protocol.
Truphone is available to download and use globally but you get an American number when setting up your account in Ireland. Then you can make and receive calls on their Truphone numbers just like a traditional phone. Your Truphone can make free calls to other Truphone users and VoIP-rate calls to anyone else.
 Handsets supported: Nokia E61, Nokia E70, and N80 Internet Edition (N80ie).
Truphone -- "What phones does it work on?"