Have you checked your AT&T account to make sure that carrier voice mail is enabled?
I am in the opposite situation, I love GV Voicemail and have been using it for years. However, in the beginning, the carrier voicemail would always interfere with GV Voicemail.
I called my carrier and told them I wanted to disable/block their voicemail. Once that was done,
GV voicemail worked like a charm.