Clicking on the link opens up a small, standard phone dialing window.
You simply dial any number for Canada or the US, and the call is placed, free of charge! It works nice and easy for me, because I already have a mic built into my laptap, but if you don't, you'll need a microphone to use with this. It seems to work fine using my laptops speakers, because of an anti echo option available.
Calling someone through Call Phone results in the number 760-705-8888 coming on their call display.
Call quality was fine for me. Voices sound a little echoy coming through my laptop speakers, but that could just be the speakers themselves, and I had no problem making out what people were saying.
Now, you may be wondering "In this day and age, when pretty much everyone has a cell phone, what use is there in a calling option like this?" Well, there are couple of instances when something like Gmail Call Phone would still be handy. If you were in a situation with poor cell phone reception, but still had internet access, calls could be placed with no issues. I was in this situation myself not too long ago. Also, the fact that you can place calls for free to anywhere in Canada and the US, you could save yourself a tonne of money in long distance charges. Unfortunately, in a blog posting I found by Robin Schriebman, in this blog posting, calls are only going to be free untill the end of the year or so. However, until they start charging for the calls, this is a great feature to be aware of, and useful many different situations.