"Talqer (pronounced Talker) integrates with Google Talk to allow users to make phone calls to normal telephones anywhere in the world!"
Prices are listed here: http://www.talqer.com/international_rates.php
Most reviews say it has a clear sound quality, eg. : http://www.customizetalk.com/forums/viewtopic.php?t=480
My digg article: http://www.digg.com/software/Talqer._Voip_on_Gtalk_-_Cheaper_than_Skype