Press and hold down the AT&T button on the Uverse remote. As you are doing this, press and release the "OK" button. The remote is ready when the mode buttons flash once.
Enter the the code 955 within ten seconds. If you wait longer than ten seconds, you will have to start all over. After you have entered the code, the AT&T mode button will flash twice to indicate that the code has been submitted.
Press the mode button for whichever device you would the volume buttons on the remote to control.