There is a service called Jabphone which allows you to connect to any phone by using Jabber Instant Messaging System with Jingle audio support (such as Google Talk). Normally it charges with good rates but they give 15 minutes of free time for any new account. So give it a try.
Every new account is credited with approximately 15 minutes of free calling time (when calling the USA – calling time varies depending on communication rates to the called region). After you’ve used up your free calling time, you can add credits to your account (we accept major credit cards through PayPal). To do so, simply login at the top right corner of the page using your account email address and the password sent to you by email.
Jabber to Phone Gateway – [Jabphone]