Google Voice's primary service provides a new phone number to which you forward calls from multiple other lines, including your home phone and cell phone. If you want to use Google Voice as a replacement only for your AT&T mobile voice mail, you can instead use Google Voice Lite. This service stores phone messages in your Google account in place of using AT&T's dial-in or visual voice mail. If you already have a Google Voice number, you can add your AT&T phone's number to your account to merge its voice mail into your Voice mailbox.

Google Voice Lite

Step 1

Visit the Google Voice website and log in with a Google account. Click "I Want to Use My Mobile Number."

Step 2

Type your phone number and click "Check for Available Options." After the window updates, click "Get Google Voice Lite."

Step 3

Press "Call Me Now" to receive a call on your cell phone. Dial the number that appears on your computer screen to verify your phone.

Step 4

Dial "1" if you want to record a voice mail greeting immediately. Otherwise, end the call -- you can record a greeting at any time later.

Step 5

Type a four to 10-digit PIN to use as your voice mail password. Check the acceptance box and click "Continue."

Step 6

Read the top line of the new window. If it does not automatically list your carrier as AT&T, click "Change Carrier," pick "AT&T" and press "Continue."

Step 7

Dial the long number that appears on the computer screen, starting with "**004." You'll see a message on your phone's screen confirming you've moved your voice mail box. Click "Continue" and "Finish."

Existing Google Voice Account

Step 1

Log in to Google Voice and click the gear icon. Click "Settings."

Step 2

Click "Add Another Phone." Pick a name for your phone and enter its number. Set the phone type to "Mobile."

Step 3

Press "Save." A box with a number displays. Click "Connect" in the box and you will receive a call on your phone. Dial the number that displays to verify your phone.

Step 4

Click "Activate Google Voicemail on This Phone" by your phone's number. Another pop-up box displays with a long number beginning with "**004." At the top of the box, you should see "Your carrier: AT&T." If Google does not automatically detect your carrier correctly, click "Change Carrier," pick "AT&T" and press "Continue."

Step 5

Call the number in the pop-up box from your phone. You will not connect to a phone line, but instead will receive a confirmation message on the phone's screen.