One of the neat features of the chat application built into the Gmail web interface is the ability to receive and place phone calls to and from your Google Voice number. And unlike the Google Voice interface, calls placed using the Gmail Talk chat widget are completely free.
Unfortunately, the new Hangouts replacement (currently not mandatory) brought over from Google + does not support the calling features of Google Talk. As such, users wanting to call phones and not just video chat with other Google + users will need to stick with the old Google Talk chat or use Google Voice and pay for outbound calls.
The good news is that the phone call features will be rolled into the new Hangouts feature eventually. According to Google employee Nikhyl Singhal, “outbound/inbound calls will soon be available.” He also indicated that future versions of Hangouts will further integrate the feature set of Google Voice (which likely refers to SMS). Unless you particularly want video chatting, I would recommend sticking with Google Talk until the new version of Hangouts is more fleshed out.