First and foremost, ensure that your phone has a stable and secure internet connection as AT&T is a web-based service that requires a solid connection to function. It would be best to connect your phone to a WiFi connection if connected to the mobile data. If the internet is not the reason for ATT voicemail service not available, please turn off your phone and turn it back on in about ten seconds. Next, dial 611 on your phone and make a test call. If the call makes through, please press 1 to dial into the voicemail system. On the contrary, if the call does not make it through, please confirm wireless coverage. Press and hold 1 to check your message if you are not able to download visual content.