The service undercuts the popular PC-to-phone service, Skype, which charges 1.2 cents to 2.1 cents per minute for U.S. calls.
It also threatens to overshadow another free PC-to-phone calling service called MagicTalk that was just introduced by VocalTec Communications Ltd.
Google hopes to make money on g-mail by charging two cents per minute for most international calls.
Observers say it may also encourage people to cut the cords on their landline telephones.
The new service is provided by Google Voice, a telecommunications hub that the company has been trying to expand.
The Associated Press contributed to this report.