Originally Posted by Flashman
Select the Phone application (press the green dial button), then the Menu button, then select Options. There you can select "Call Forwarding" and select "Forward All Calls" instead of the original "Forward Unanswered Calls", making sure to leave the entry as "Do Not Forward". I haven't tried it, but in theory your phone should ring until whatever limit AT&T has for ringers (there is a limit...).
You can also change the number pressing #1 calls by going to the "Voice Mail" option in the same section.
at&t disables the modification of voicemail features via the handset. As an at&t customer, if you wish to disable voicemail, you must call customer care and have the feature removed.