Then scroll down to the Tethering & Internet Settings, then choosing your country and provider. This works for both AT&T and T-Mobile, and will let you install the appropriate configuration. Just be careful though, that AT&T doesn’t officially sponsor this and might charge you extra for using tethering while you’re not supposed to.
And if this disables visual voicemail on your phone, just go and reset your network settings, and it should be fixed. If that doesn’t work, try updating your phone with an older version of the AT&T carrier settings.