The "Make a call" button was added to the new Google Voice web UI last week, along with an updated release of the Android app to include the same function. This was in response to user feedback, complaining about its absence. The "Make a call" button performs the same function as the red and white CALL button on the old, "Legacy" UI. As before, it requires that the user select one of their verified forwarding (or "linked" in the new UI) 10-digit telephone numbers, or Hangouts, to use as the target for the call. As with the old UI, it does not support Chat.
Yes, you can call any number, not just numbers in your Google Contacts.