Video calls among two or more Google+ users was the big draw when Hangouts launched, but now Google has flipped the switch the allow Google+ users to make phone calls to almost any phone number, not just within the Google+ ecosystem. That means that users will be able, for example, place a voice call from their computers and reach friends or family on their land line or cell phone.
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The feature is limited to outbound calls and cannot accept incoming calls to your Google+ account. It's also currently desktop-only, and isn't compatible with Google+ on mobile apps.
Calls to phone numbers in the U.S. and Canada are free, and callers can buy credit to dial out to international lines.
To place a call, click the "Invite" button in Hangouts, then the "+telephone" link. After entering the number, hit "add." The dialer currently stores one number at a time.
The Google Voice platform powers the entire operation, though the feature bears the Google+ Hangouts name. You can similarly place calls to land lines and mobile phones using Google Voice's integration into Gmail.
Calling out with Hangouts is a third high-profile experimental feature that Google has pulled into the mainstream product, along with screen-sharing and support for Google Docs. Source : Source