The recently rumored voice calling option is already live on all of my Gmail accounts, not sure if everyone else is seeing it, or if users with existing Google Voice accounts got it added automatically. At any rate, voice calling works fairly well. Though in my tests people on the other end complained of frequent static even though I could hear them just fine.
All my out going calls showed up as coming from my Google Voice number, so I’m making the assumption that you’ll need to have a Google Voice account for this to work.
Making a call is simple, just select Call phone from the Chat box on the left hand side of your Gmail window and a dialer will popup.
Start typing either a name of a contact or there phone number and Gmail will provide auto-complete search results, since I already use Google Voice for my business this worked amazingly well because all my contacts already had phone numbers attached. If the number your dialing isn’t attached to a contact, just enter it normally.
Click on a name or press enter to start dialing.