Try looking here, especially under 'Reset your voicemail password.'
http://www.att.com/esupport/article.jsp?sid=KB92829&cv=821#fbid=xoR3wTS1mLY
It's strange that the system could not detect your phone number... Are you sure that your phone is still active? You can do this by dialing *777# or by logging into your Pay As Yo Go online account here:
https://www.paygonline.com/websc/index.jsp
http://www.att.com/esupport/article.jsp?sid=KB92829&cv=821#fbid=xoR3wTS1mLY
It's strange that the system could not detect your phone number... Are you sure that your phone is still active? You can do this by dialing *777# or by logging into your Pay As Yo Go online account here:
https://www.paygonline.com/websc/index.jsp