I have just posted a new tutorial that uses the Ribbit Phone API to add VoIP capabilities to a Flex application.

Here is the link: Adding VoIP to Your Flex Application

This tutorial builds a quick sample application that allows you to call a telephone from within your Flex application using your computer's microphone and speakers.

Ribbit Phone has made it surprisingly easy to add VoIP functionality with minimal code and effort. The call quality isn't quite as good as Skype, but considering it's running completely through the Flash player I would say it is still quite impressive.