Presently Google voice account is through invitation only . Today Google Voice blog has announced an exciting offer for student considering the difficulties of sending and receiving voice mails in the conventional way.
If you are a student, then goto http://www.google.com/voice/students and enter your email id (it must be ends in .edu domain). Google says ” you will get your invitation within 24 hrs”. Only one invite will be sent per email id.
Presently Google Voice service is available in US only. People of other countries have to wait (how long?).