You can receive Google+ Hangouts invites and voice chats (from one computer to another) in Chat. Just download the Google voice and video chat plugin and sign into Chat to get started.
If you don’t have a Google+ account you can start or join a limited hangout, which only includes the option to hang out with 1 other person. If you’d like access to all Hangouts’ features including the ability to hang out with up to 9 people, share your screen, and use cool Hangouts’ apps, you can upgrade to a free Google+ account at plus.google.com.
Whenever someone calls you in chat, you’ll hear a sound notification and a chat window will appear. To answer a voice chat, click the Answer button in the chat window or, click Join Hangout to accept your hangout invite. You can ignore the call by clicking the Ignore button.
If you miss your call, don't worry. The chat window will open, with a message saying who attempted to call you.
Your contacts can still place calls to you even if you have a status of Busy. You cannot opt out of taking voice or video calls while still receiving normal text chats.