I just tested it and it works. Make sure gv is set up then dial *38 send and wait for it to beep then hang up. Then dial*28 then your area code and number of your google voice then send. Listen for the beeps then it will hang up on you. You're all set now. If someone calls your sprint number and you let it ring or reject it, it will go to gvvm. If they call gv number, if you let ring our reject call, it will go to gvvm. Don't worry about the voicemail setting for the phone. Just get gv app set up completely on phone, make sure the gv account settings are straight from a regular computer online, tell your phone to conditional call forward, then everything should work like normal (minus the choose voicemail setting that isn't there on the phone).
One last thing, make sure the plan you have includes conditional call waiting for free or if it doesn't you're gonna get charged for it. If someone calls gv #, that is considered a landline and won't be anytime mobile. If you call out through gv, that's considered you calling a land line so that won't be included with anytime mobile minutes. I have the kitchen sink plan so that doesn't go against me.