Google new Hangouts messaging service is quite impressive — it takes Google's old and neglected Talk service and upgrades it with always-on communication, support for images, video calling, and much more. But for all of the improved service that Hangouts offers, if you upgrade the Google Talk service within Gmail to Hangouts (by selecting the "Try the new Hangouts" option in Talk's menu), you lose some significant features — namely the ability to set your availability status and, more importantly, the ability to place or receive Google Voice calls from within Gmail. Essentially, the new Hangouts removes the option to host a Google Voice call on your computer, something that many users find convenient and necessary in their day to day workflow.
Users in Google's support forums first reported the feature's disappearance last week, and The Verge has confirmed that it is not possible to place or receive calls in Gmail once you have upgraded Talk to Hangouts. The only option for users is to initiate a call from the Google Voice web app, which will ring one of their connected phones before routing the call to the other party.
Google added this feature to Gmail back in 2010, and for many Google Voice users, it's been an indispensable tool for placing outgoing calls from their computer. It didn't hurt that Google has offered completely free calls from Gmail to phone numbers within the US since the feature launched, either.
Users might want to hold off on upgrading to Hangouts
Fortunately, all is not lost, as Google has made it possible to revert back to the old Google Talk service in Gmail if you have already upgraded to Hangouts. This does restore the ability to host Google Voice calls on your computer without having to route the call to another phone before it connects. It's also worth noting that although advanced Hangouts features don't work with the XMPP standards, users who upgrade to Hangouts still do have limited support to use other traditional GTalk apps like Trilian and Adium — basically Google Talk and Hangouts still talk to each other, albeit in a limited fashion.
Since Google has unified its various messaging platforms and shifted around some of the teams that run them, it has promised to add some of Google Voice's features (along with SMS support) to Hangouts in the future. We don't yet know if this specific feature will be making the cut, but we're holding out hope that it does. For now, the best option for those that use Google Talk to place calls from their computers is to hold off on upgrading to Hangouts — and pray that the option doesn't disappear completely in the future.
We have reached out to Google for comment on the matter and will update this post if we hear back.
Update: Google product manager Nikhyl Singhal acknowledged in a post on Google+ today that Google Voice is missing when users upgrade to Hangouts, but said that the update does "support inbound calls," and added "we're working hard on supporting both, and outbound/inbound calls will soon be available."