Both Yahoo and Google are having Mail, Messenger, Voice, Video calls services. Google is having so many free services like Orkut, Blogs, Ad sense, RSS , Feeds and many more.
Now Google voice is having free phone service in US and Canada. You need to have mobile or Land line phone to call any US and Canada number. In order to call India and other countries you need to pay for the calls. I hope soon Google voice will become popular all over the world as they are making everything free
Using Google voice service you can call India at the rate of 0.06 cents per minute and the same service by using Yahoo is 0.079 and Calls to US and Canada is free and you can call US number.
Try to register in Google voice and you can call the US and Canada number for free of cost and also SMS too. Soon Google voice will be free in other countries also.