Google's gmail phone works for me

Read the news about the google phone in gmail yesterday. It said the offer is for US only and free for US and CA by the end of year.
I curiously tried to install the feature. There's no "Call phone" button displayed in chat panel when I restarted browser. Can't figure it out.
However, when I get to the office and open the browser, The button is shown. Can't wait to try it even I don't have a head mic. It works.
I probably won't use it much, but it still a nice feature. I hope google can integrate all voice feature with gmail.