I saw tests of it in 2015 in the Local KP. At the time, it was obviously visible to some and not others.
That being said I had not seen it broadly integrated into the Local Finder before. Could be I am just blind and its been there forever but since it new(s) to me, I am documenting it. One more data point that Google can collect to learn how customers interact with your business; call length and other meta data. While the convo itself is encrypted I am sure that Google extracts other data points as well.
When you click on a phone number from within the Local Finder or the Knowledge Panel on the desktop, I now regularly get shunted to Google Hangouts for call completion:Desktop Click to Call via Hangouts from the Local Finder & KP by Mike Blumenthal