The answer is so simple; I wonder if this would also work for the Verizon phone as well. In short, just hit * during the outgoing message and it will then ask for the voicemail passcode. Here is where I found this on a forum:
http://forums.att.com/t5/Voice-Mail/Is-there-a-central-number-to-call-for-at-amp-t-voicemail-access/td-p/1553946
Here is an interesting alternative suggestion which I didn't get around to trying:
https://www.customerservice.att.com/assistance/faq/qa.jsp?faqCategoryId=local_accessvm_faq
