Unlike Google Voice, it dos not make a call to one of your phones before connecting you to the caller, but rather connects you directly to the microphone and speakers (or headset) on your PC.
To make a call, just click "Call Phone" that appears on the left hand column, underneath Chat.
A dialpad appears. You can either type the number using the keypad, type in a number in the search box, or search your contacts by typing into the search box.
Without Google Voice, you cannot receive calls from the phone system. If you do have GV, you can click Settings, Voice Settings, phones and you'll see an option for turning on Google Chat.
The calls are free in the US and Canada. Overseas calling is pretty cheap too. It's ony two cents a minute to the UK. Our beloved AT&T nicked me $2.48 a minute a few years ago.
Phone calls in Gmail are nice, but not yet the be all end all.
It doesn't replace the need to go to the GV homepage. GV still offers text messaging. I suspect that will be added here soon.
And while it works on PC's and Macs, don't look for it soon on your iPad. “We do not offer this feature on mobile browsers, and right now we have no plans to do so,” according to Googler Randall Sarafa, quoted in BusinessWeek.
Those of us who had Gizmo5 accounts before Google bought Google could have Google Voice calls come to our Gizmo5 client. Once nice ability there was to record calls. GV lets you record incoming calls, but not out going calls.
There are some issues. VoipWatch says there is some problems keeping the Gmail and Google Voice windows all in sync.
There are some nice things though.
Since you can create as many Google Accounts as you need, Lifehacker.com talks about using a GV account as a special Emergency Contact mechanism. I have to think about that one some more.
Back in July there was a rumor Google had an updated Gizmo5 client ready to work with GV, but higher ups wanted a web based only solution, not a PC client. Apparently this Gmail ability is that web based client. I do hope though Google comes up with a smaller web window to act as a floating client. I have placed calls, and then clicked away from my Gmail window to look up something whle on the call, and have to go back to the Gmail window to end the call. Having a little floating web app would be nice.