In my prior experience with Android phones, when you setup Google Voice, it offers to change your default voicemail provider. It would send you to the Settings -> Call Settings page. Normally, you'd have a "Voicemail Service" option to select a different voicemail service than Sprint's default. Here I'd select Google Voice.
Unfortunately, the option is blocked or missing on my LG Optimus S. I'd like to see this fixed in a maintenance update.
Message was edited by: KMBULEBU
You just have to set it up using conditional call forwarding.
Dial *28nnnnnnnnnn where the n's are your Google Voice number. That'll send ring-no-answer and busy calls to your Google Voice box free of charge.
You can read more about the Call Forwarding options at:
"Sprint is the most improved U.S. company in customer satisfaction, across all 43 industries, over the last six years." - 2014 American Customer Satisfaction Index