Excerpted from a recent New York Times article:

CallingAmerica.com is a new ad-supported Web service that lets you make phone calls to any number in the United States from any computer with a Web browser and a broadband connection.

The service, which begins Thursday, requires you to view a short ad before you place the call. The audio is played through the browser, and the site uses your computer’s microphone to transmit what you say.

There is a limit to the fun, however. Unregistered users can make calls of up to two minutes; registration — which is also free — gives you unlimited calls of up to 15 minutes. While it’s not as versatile as other Internet calling services like Skype or Vonage, it is enough to make a few quick calls while on the road without racking up long-distance charges.

Most important, the service works domestically and all over the world.