The free Virtual Number was being included in the initial sign-up.
If you signed up recently, all Virtual Numbers must be paid for.
An email to support@voipo.com should tell you, if a free one is available for you.

http://forums.voipo.com/showpost.php?p=7775&postcount=1

The Founders Plan for the Beta testers, may or may not still include a free VN now?