SBC residential voice mail comes with a default password that is snailmailed to the new customer and, presumably, different for each one.  The scheme used to generate these passwords is not known.

You can reset an SBC residential voice mail password using the following procedure:

(from SBC literature)

Question: What if I forget my Message Center password? 
 Answer: For your safety and privacy, we do not maintain your password, but we can reset it for you. To reset your password, call our automated system at 1-800-675-9005. After entering your area code and phone number, select option 3 and follow the voice prompts.