You have to make sure that you are logged into the google voice website
https://www.google.com/accounts/Serv...Z-F3Qse1v2WROA
Once your logged in, go to settings, then under PHONES-make sure your cell number is listed as a registered phone. If it is, click edit. Under Forwarding Options, hit activate. It will have specific directions for your carrier. But like the other posts said..it should be *28(then your Google number) and send/call.