The companies did not reveal terms of the deal, which was announced Thursday following rumors earlier this week that the acquisition might happen.
In a blog posting about the deal, Google said that while current Gizmo5 users can continue to use the service, no new customers can sign up for now.
Very little information remains on the Gizmo5 Web page. The service is similar to Skype's in that it allows users to make low-cost or free calls using a software client on their computers. Gizmo5 also offers a client for mobile-phone users.
The acquisition follows Google's purchase of Grand Central more than two years ago. Fans of that service began to worry it might die at Google since it took about two years for Google to begin accepting new customers for the service, now called Google Voice.
Google Voice offers users a variety of services including transcribed voicemails and low-cost calling. Users can also get a new phone number and use that same number on multiple phones.
The service has been the subject of some controversy. It is the subject of a U.S. Federal Communications Commission inquiry, launched as a result of complaints that Google blocks some calls.
Gizmo5 and Google Voice are already integrated; Google Voice customers can include a Gizmo5 account as one of their phones.
It is the fourth acquisition Google has announced this year. Earlier this week it said it would buy AdMob for $750 million.
