Well, my phone still says ATTWS despite what the (cingular) rep told me, but even tough they are one company, they are still far from being integrated with the services.
For example: my wife has a Cingular line. On her plan, she has unlimited Mobile to Mobile minutes, which means that any inbound/outbound call from/to a Cingular AND ATT are included in that. But, me, as an ATT customer, even tough I have the plan for Mobile to Mobile, it only covers other ATTWS numbers.....

and this will likely be like this for a couple more months.
As for your question... leave the ATT FW, no need to upgrade to Cingular, unless you want to....
And as for Voice mail number.... should be your mobile number...