Ensure you check and enable "google chat"
Do manage google voice preferences within the google voice application
-Bee it how it forwards the calls/ notification or transciption ( albeit imperfect ) to your email.
  Also See the  details   on the right   and below        ( very important )    ...........................>>>
 Access this page from your gmail account ( using google chrome preferably  ) or https://www.google.com/voice?pli=1#inbox
The forwarding assiged number is not relevant any more and should  be un-checked ( if you are not in the US).
Note: the Forwarding number is only relevant for those that are physically in the USA/Canada ( those starting with a country code +1 )
               ensure Google chat is checked by default 
                                                                                                         All Access numbers / used for verification need NOT be checked
        The forwarding number is  checked during call back verification . After which you may leave it unckecked.   
        Call cellcentric  will pull it off from you.
The  Forwarding numbers have some tricks ( beyond the scope of discussion here ) that can help to redirect calls via 
 free pbxes  ( like pbxes.org)                
 Repeated for clarity
The Forwarding  #  when it is a actual physical number as used within the USA / canada and then  only relevant to those physicall present in the USA / canada ..and if you wish calls to google voice directed to it .
Note It is not "usually"relevant  for those who are outside USA /CANADA.   
Forwarding  should be left Unchecked ( normally) in most cases for those with a google voice and you are not in the USA /Canada. 
Note where I have stated usually and normally- as there are exceptional ways it can be used . 

 get Groove Ip  ( click )
Yes, Voice without using minutes
GrooVe IP uses approximately 1 MB /minute during a call. Hence Either be on a Wifi or a good DATA plan

And then use it with your google voice acount.
Grove IP can Even be installed and managed even on a Tablet.
Groove Ip is perhaps the best, anytime better  than many others like BRIA or SIP Acrobits which cost ($) or more.
(Accurate, at this point when this is written/ last edited ) .-October 2012
Important  Message
This application may soon NOT work with Google voice , once google really pulls support over Jabber (XMPP) protocols ... as they said it would  pull support by June 2014. 
As on 1st Week of December 2014 , it was still working