This new build is for windows users only i think Mac users will disappointed again 
New feature:Free Voice
You can call toll-free U.S. numbers that start with area code 800, 888, 877, or 866. This calling features are available to users only on these versions of Yahoo! Messenger: U.S., Yahoo! Telemundo (in Spanish), Spain, France, Germany, Hong Kong, Singapore
If you’ve never called a land line phone using Yahoo! Messenger’s Phone Out Yahoo! Voice service, calling a toll-free numbers is a great way to try the service for free without having to sign up first. Click a phone number below to make a toll-free call right now. (You must be signed into Messenger and using version 8.1).
Yahoo! Messenger blog also says It includes improvements in voice quality and sign-in as well as fixes for minor bugs with the new Microsoft Windows Vista operating system that launches on January 30.
Hope i can use this one cause of bugs at the latest version i am still using v7
more info
Download latest Yahoo! Messenger