I would suggest contacting Uniden rather than AT&T. I included their contact information below as well as 3 issues that may be going on here.
Uniden Technical Support
M-F 8:30am-11:45pm CST & 2:30pm-4:45pm CST
Ask a Question: http://mycusthelp.net/UNIDEN/_cs/QuestionAsk.aspx?sSessionID=&
1. Are you getting messages? If not, the voicemail may have set to "Announce Only." This means the Answering system will answer the call, but will not allow callers to leave a message. If so, there may be an LED "A" lit up on your base. You can change this setting in the Answ setup --> record time.
2. Does it occur after a particular number of rings? You can set the voicemail to answer after 2, 4, or 6 rings. This may be the problem.
3. Does the phone ring at all? It is possible the phone is in Silent Mode. You can set silent mode to last anywhere between 1-9 hours or keep it on all the time. If it is set to a particular number of hours, the phone will exit silent mode automatically. If not, you need to hold # to deactivate it. I don't
think you can set silent mode to turn on automatically on a schedule, but this is something you could ask Uniden about.
I hope that helps. Good Luck!