The following settings will allow you to use the Voice app on your phone to read/listen to voicemails, while leaving phone calls and text to the default apps:
(You must have a GV number obviously, as what you are doing is just forwarding your voicemails from Sprint to Google)
1. If you've integrated your GV and Sprint number together, go into the GV settings on your computer and separate them.
2. On the computer, click the "Activate Google voicemail on this phone" link and follow the steps there, dialing *28+your GV number from your Nexus
---(Optional) Also on the computer, go into the Voicemail & Text settings tab and uncheck both the Voicemail Notification checkboxes, you're already going to be getting notified from the Voice app on your phone
3.Now on your phone, sign into the GV app
4.Click on the "Sync and notifications" and make sure "Synchronize Inbox" and "Inbox notifications" are both checked.
If you did the Optional step above, now when someone leaves you a voicemail you won't bombarded with texts, emails and voicemails. The Voice app will just leave a notification icon.