Let me explain a little better now that I'm on a computer typing. I've been using it for a few weeks now and I love it. Comes in handy in the house when your on the computer/ lack of service where you are (with computer access). Also, the girlfriend wanted to borrow my phone for school one day so she could use it to take pictures. I was able to set her phone up to foward all my calls and text messages to. (Could go on the website to respond using my number; it was a feature phone.) And again, give it a shot. Mess with it. If you don't like it disable it and all is normal again.
First what you do is go onto google voice. It's either upgrade account or I believe when you add a Sprint phone it will have an option to use for google voice.
You put in the number you want to use for THAT google account. (If you have other google accounts you could use your other numbers for those accounts; but, it's only going to effect the number you chose). You enable the voice + sprint and wallah. Now your google voice number is your sprint number. All your calls and text messages will go through google; then, to you. And vise versa.
Now, I'm not sure how it works without the GV app on your phone; but, here's what it does with GV app.
You download the google voice app. You pick a google account you want to use (You can sign out later and use a different one; IE, if let's say someone is with you and there phone is dead and they want to send a text message they could sign in with their account if it's synced on your phone.) From there it will prompt you to set up your voice mail by calling a number. (I believe this happens; this is what it did before this came out).
Now when someone calls you - you won't even notice a difference. It comes through like normal. All your call logs will also be saved on google voice website. Missed called, placed, etc. You voicemail will come through there; which, is visual. And I've been using for months now. Better than the one on my epic anyways. And you can play them and everything from the computer.
When you receive SMS it will no longer (at this time) come through your regular messaging app. It will come through GV. MMS will go through your regular messaging app because GV does not support MMS. If you would like to get SMS regular like you've been doing either under the phones tab on GV Website or on your GV app under notifications you would chose receive SMS on this phone. However, there is no way to disable notifications through GV app for SMS so you'll get double.
Go on the website, mess with some of the settings. Which phones to ring (At this point if that's all you've done you'll have that phone and gtalk.) when to ring, , etc.
Currently I have it set up to ring gmail and my sprint phone. I disabled text message alerts for voicemail (I guess that's for feature phones) and all the extra things you'll see when you go through the settings. I have a third phone on their but it's unchecked so it doesn't ring. You can check and uncheck anyphone you have on there to ring or not ring.