TO THE THREAD-STARTER: How you ultimately do it will be entirely up to you, but here's how I do it, step-by-painful-step, and it works magnificently for me and my particular purposes (all of which I fully explain, below)...
Download and install the Google Voice app to the phone.
Launch the Google Voice app on the phone. You'll land on the "Welcome to Google Voice" screen.
On the "Welcome to Google Voice" screen, click the "Next" button at the bottom. You'll land on the "Sign in to Google Voice" screen; and your gmail account associated with the phone should be the only choice under "Select an account to use with Voice:".
If so, then make sure that that one and only gmail account has a green dot to the right of it, and then click on the "Sign in" button at the botttom.
You'll land on the "Access Request" screen. Click on the "Allow" button at the bottom.
It will then slide back to the "Sign in to Google Voice" screen while it signs in, and then you'll land on the "Set up Google Voice" screen. Click on the "Next" button at the bottom.
You'll land on the "Making calls with Google Voice" screen. Choose whatever you want, but I always select "Ask every time I make a call." Then click on the "Next" button at the bottom.
I never want my Google Voice to be my voicemail. I don't use the voicemail that
You'll land on the first (of two) "Set up voicemail" screens. Click on the "Next" button at the bottom. You'll then land on the second "Set up voicemail" screen which offers to set up Google Voice as the phone's official voicemail. I never do that. I don't use the voicemail provided by AT&T and tied to the cell phone's actual AT&T number, either. Instead, I have a unified messaging system where all voicemail, no matter the phone in my life, goes (and I've keyed its telephone number into the phone's voicemail section). And so, for me, at least, I just SKIP what this second "Set up voicemail" screen wants to do. If you agree, then press the "Skip" button at the bottom of the screen.
You'll land on the "Inbox syncronization" screen. Read what it says so you're clear about it, and then press the "Finish" button at the bottom.
The Google Voice app will then download the contents of the Google Voice inbox, and if there's anything new there, it'll make the default notification sound.
Press the phone's "Menu" button, then press the "More" choice, then press "Settings." You'll land on the Google Voice settings screen, with "General" at the top.
Press on the "Sync and notifications" choice. You'll land on the "Sync and notifications" screen, showing "Synchronization" at the top.
Make sure "Background data" is enabled.
Note (but don't press yet) the "Receive text messages" item, in the "Notifications" section. It should be defaulted to "Also via the messaging app," with the "Text Notifications" item, immediately beneath, grayed-out. Before changing any of that you need to decide how you'll be notified when you receive an SMS via Google Voice; and here's what you need to know about that...
The default settings just described will cause any SMS messages received by the Google Voice number to show-up not only in the phone's Google Voice app (where you'd expect it to appear), but also in the phone's normal SMS area; and so whatever notification (the noise the phone makes, and/or the icon that shows-up on the status bar) the phone makes when you receive an SMS sent to your phone's regular (non-Google-Voice) number is the same notification (sound and icon) you'll get if you receive a Google Voice SMS; and said SMS sent to the Google Voice number will appear in the phone's stock messaging app, alongside SMS messages sent to the AT&T-provided (non-Google-Voice) phone number. I, personally, don't like that, because I want the SMS messages sent to my Google Voice number to make a different noise on the phone; and I don't want to be confused about which SMS messages came in to the phone's normal AT&T-provided number, and which ones came-in to the Google Voice number. But that's just me.
If you agree, though, then press the "Receive text messages" item, and select, from the pop-up dialog, the "Only via the Google Voice app" choice.
At that point, the "Text notifications" item, immediately below, lights-up, and defaults to check-marked status, meaning that any SMS sent to the Google Voice number will cause a notification on the status bar. Whether or not you want that is up to you, but leaving it checked, plus the change described in the immediately-preceding paragraph, will cause all incoming SMS to the Google Voice number to only be visible in the Google Voice app; but it will nevertheless cause a notification icon to appear on the status bar.
Setting things up this way is useful because you don't want to be replying to Google Voice SMS messages from the phone's default/stock messaging app, which is a mistake you could easily make if incoming Google Voice SMS messages appear there as well as in the Google Voice app, instead of only in the Google Voice app (the latter of which being how we just configured things); and the reason you don't want to be replying to Google Voice SMS messages from the phone's stock/default messaging app is that if you do, it will be your phone's AT&T-provided number, and not your Google Voice number, that will appear in said message's "From:" field. Assuming you agree, let's continue...
If you're like me, and you turn off as much vibrating throughout the phone as possible (to save battery), then you'll uncheck the "Vibrate" item.
Whether you allow the status light to blink when a new Google Voice SMS message comes in is also up to you. If you want it, then leave "Light" checked; and if you don't, then uncheck it.
If you'd like to give incoming Google Voice SMS messages their own noise (which is one of the coolest features, especially if you've opted not to allow incoming Google Voice messages to integrate with and appear among SMS messages sent to your phone's regular AT&T-provided (non-Google-Voice) number), then press the "Select ringtone" item, and choose whatever noise, other than the phone's default notification, that you'd like to hear whenever a Google Voice SMS message arrives.
Then press the phone's "Back" button to return to the "Google Voice" settings screen; and before leaving it, just for grins, scroll down to the "About" section and verify that the right number is appearing beneath "My Google Voice number." If your GMAIL login worked properly; and if your desired Google Voice number is correctly tied to that, then the correct Google Voice number should appear there... but just check to be sure. If it's not the right Google Voice number, then pretty much nothing is going to work right; and if that's the case, then before continuing, you need to go figure out what's wrong and get it all squared away. If everything's okay, then let's continue...
Press the phone's "Back" button again to return to the Google Voice app's inbox.
To learn more about how to actually use the Google Voice app to both read and reply to texts (and also to see, and even listen to, through the phone's speaker, any voicemails sitting in the Google Voice inbox) using the Google Voice app, press the phone's "Menu" button, then the "More" choice, then the "Help" item. You should, however, be able to pretty much figure out how to use it just from playing with it a bit. If you have no SMS messages sitting in the inbox to which you can reply, as a means of teaching yourself, then send Google Voice number an SMS from your phone's AT&T-provided number, using the phone's stock messaging app. Then once it arrives in the Google Voice inbox, press on the SMS and you'll see how you can press on the "Type to Compose" field at the very bottom to launch the keyboard and reply.
If someone leaves a voicemail in your Google Voice inbox, you can press on the message (which should also be transcribed... that is, assuming Google Voice isn't so busy that it hasn't yet had time to transcribe it), and you'll notice a "Play" button and progress bar at the very bottom. Pressing the "Play" button will play the voicemail message through the phone's speaker.
For the rest of the things you can do in the app, I refer you, again, to the "Help" mentioned herein above.
To leave the Google Voice app, just press the phone's "Back" button however many times it takes to back-out of the app, and return to a homescreen.
To make Google Voice work best with your AT&T phone, go to your desktop or laptop computer and login to your Google Voice account through the browser (I recommend you use Chrome, only because of an extension I'm also going to recommend, herein).
Once logged-in to your Google Voice account via your desktop/laptop browser, find the word "Voice" in red letters, just above the "Call" and "Text" buttons, and then move your eyes horizontally across the screen from the word "Voice" until you see the button with the gear on it at the far right (you'll be told in a little pop-up that it's the "Settings" button, if you hover the mouse pointer above it). (I brought you to that button that way because I don't want you to press on the button with the gear on it higher-up on the page... the one that's tied to your overall Google account, and not specifically to your Google Voice account.)
Click on that "Settings" button with the gear on it (that's stright-right of the red-lettered word "Voice"), and select "Settings" from the little drop-down menu. You'll be taken to the Google Voice settings, landing on the "Phones" tab.
If you use Google Voice the way I do, then you'll want all calls to your Google Voice number to auto-forward to your cell phone. I, in fact, give-out my Google Voice number as my cell number to everyone. No one (except my wife, and AT&T, of course) actually knows my phone's AT&T-provided number. I do this for the same reason I tell people to never use the email address provided to them by their Internet Service Provicer (ISP), so that if they change ISPs, they don't have to change email addresses (and have everyone they know change same in their address books). Giving-out only the Google Voice number, and never the phone's actual carrier-provided number allows you to change carriers, and get a whole new carrier-provided number, and still keep the same Google Voice number. I also like giving-out only my Google Voice number because it offers such effective call-blocking capabilities if I ever start hearing from someone whom I don't want to hear from, or who won't stop calling me after I've asked them to, etc. Using Google Voice for all incoming calls, at least, can be very handy, indeed.
Whether or not you also use Google Voice for all your outgoing calls is up to you; but if you use your phone's regular AT&T-provided number to call people, yet you don't want them to know your phone's real (AT&T-provided) number, then you need to remember to block your phone's AT&T-provided number from showing-up in the caller ID of people you call on it by pressing the phone's "Menu" button (from any homescreen), then pressing "Settings," then pressing "Call settings," then "All calls," then pressing "Show my number" and then selecting "Hide" on the dialog that pops-up; then press the "Back" button however many times are needed to get back to a homescreen.
Anytime you make a call to someone who doesn't accept Caller-ID-blocked calls, then you would simply make the outgoing call via Google Voice, and not via your phone's AT&T-provided regular number. And, of course, whenever you use Google Voice to make a call, then either your WIFI connection to the Internet (if you so happen to be connected at the time), or your 3G/4G data plan, and not any of your phone's regular AT&T-provided number's minutes will be used. The sixty-four-thousand-dollar question has always been whether it's cheaper (in terms of money paid for data) to make a call via VoIP via Google Voice, than it is to make that same call using the phone's regular minutes using the phone's regular AT&T-provided number. That's someting you'd need to figure out. Certainly if the Google Voice outgoing call is made while your phone has a free WIFI connection, then the point's moot; and the truth is that VoIP calls, just generally, tend to use so little bandwidth that it would probably be something to worry about if the VoIP call went out via Google Voice over your regular 3G/4G data plan only if you have the 200-minute-per-month plan, or if your 2GB-per-month plan is about to be used-up before the billing month is overwith. But only you can decide if any given call should go out over Google Voice (and use-up a little of your 3G/4G data plan), or your phone's regular AT&T-provided phone number (and use-up some cellular minutes).
And being able to make that simple decision for any given call is precisely why I specify (and suggested, however many paragraphs back, that you also specify) that whether the phone call should be made via Google Voice, or the phone's regular AT&T-provided number, should be asked for each call. It's only one extra press after dialing the number, and it's well-worth it to have the choice... or so it is my opinion, at least.
In any case, getting back to the "Phones" tab in Google Voice's "Settings" area, in the browser on your desktop/notebook computer, if you use Google Voice like I do, you'll forward all incoming Google Voice calls to your cell phone; but you will not put a checkmark next to either "Receive text messages on this phone," or "Notify me of new voicemails via text" because both of those tasks are well-handed on your phone by the Google Voice app.
And unless you want Google Voice's voicemail to take-over as not only the voicemail used for calls coming-in to your Google Voice number, but also voicemails coming-in to your phone's regular AT&T-provided number, do not click on the "Activate Google voicemail on this phone" link.
Just, instead, simply forward all incoming Google Voice calls to your cell number (which you'll have to verify) in your desktop/laptop browser; and then click on the "Calls" tab (two tabs over to the right of the "Phones" tab), and in the "Caller ID (incoming)" field, decide whether it is the caller's number, or your Google Voice number, that you would like to appear on your phone's incoming caller ID whenever a Google Voice call is forwarded to your phone. The benefit of having your Google Voice number appear is that you'd know that it's a calling coming-in on your Google Voice number, and not on your phone's regular AT&T-provided number. In my case, since I never give-out that AT&T-provided number, I already know that unless it's my wife or AT&T calling, the call has to be coming-in on my Google Voice number, and so I select "Display caller's number." But you may choose otherwise... your choice.
Just FYI... note, just for reference, the choice beneath: "Caller ID (outgoing)." Remember that I earlier said that you only want to reply to incoming SMS sent to your Google Voice number via the Google Voice app (and not via the phone's regular SMS messaging app) so that it is your Google Voice number, and not your phone's regular AT&T-provided number which appears in your reply text's "From:" field. Technically, if you select "Display my Google Voice number" in the "Caller ID (outgoing)" field, any texts sent from the phone's regular, stock messaging app which is in reply to an SMS message sent to your Google Voice number will, because of that setting, show your Google Voice number, and not your phone's regular AT&T-provided number, in the text's "From:" field; and in the future, the same will work even for voice calls. However, in my experience, that's not reliable; and so even though you should select "Display my Google Voice number" in the "Caller ID (outgoing)" field, just as a matter of good practice, I would never actually rely on it; and, instead, simply always reply to texts sent to your Google Voice number via the phone's Google Voice app.
Whatever other things you configure on that "Calls" tab is your business; and whenever you're done, scroll down and click on the "Save changes" button, at which point you'll be returned to the Google Voice inbox in your desktop/laptop browser.
Whatever else you decide to do with Google Voice, if you set everything as I've just described, all incoming SMS messages (texts) to your Google Voice number will land in your Google Voice inbox, and you'll be notified of them by the Google Voice app on your phone (using whatever noise you configured, and both flashing the light on the taskbar, and placing a notification message there, if that's how you configured your Google Voice app (or not, if you didn't). And then you'd simply launch the Google Voice app to read the text, and reply thereto... from the Google Voice app, and not from the phone's stock messaging app.
Since any voice calls to your Google Voice number will auto-forward to your phone, it will likely be your phone's default, AT&T-provided voicemail that will take it if you don't answer; however, that said, if your AT&T-provided voicemail doesn't pick-up sufficiently quickly, then it will be your Google Voice number's voicemail that will take the message. If so, then you'll be notified of that, too, by the Google Voice app. Having voicemails in two different places can be a pain, but the Google Voice app's ability to listen to the voicemail in the Google Voice inbox without having to dial-in to your Google Voice number makes it, all things considered, pretty painless. Obviously, it would be best if all voicemail is in the same place; and one way to assure that is to change how I've herein prescribed that you configure things such that your Google Voice number's voicemail becomes your only Voicemail... in other words, it takes voicemails both from calls made to your Google Voice number, as well as calls made to your phone's regular AT&T-provided number. There can be some benefit to that. Again, though, I don't have it setup that way because of my unified messaging where ALL of my voicemail lands, no matter which phone number is called. But that's an advanced thing which has nothing to do with why you started this thread.
Hope that helps!
Gregg L. DesElms
Napa, California USA
gregg at greggdeselms dot com