It appears that the set-up software for the AT&T Unified Messaging system that is displacing our current voice mail does not know what to do when the computer is a Mac. As was the case with their attempt to compete with Comcast, this is another example of their trying to roll out a technology with absolutely no idea who their users are or what those users do. It is also a reminder that AT&T is not the company formerly known as Ma Bell. Rather, the AT&T brand was purchased by SBC, famous in at least one city where I lived as the "Stupid Solutions People!"
Alternative explanation: AT&T wants to shut down wired service and drive its customers into wireless whether they want it or not.