The lines going into the PBX are seen as analog lines. You need to find out what extension is the auto-attendant extension. Most times it is assigned as extension 7300. If you go to System Program, Exit (or Start depending upon release), then Extensions, More, More again, then Cover Delay, enter the extension to change (7300 in this example), then Primary and enter, then you can change the number of rings and enter.
The important part is finding out what extension is programmed as your auto-attendant. If you log-in as administrator to your voicemail and then go to mailboxes and enter 7300, see if it exists as auto-attendant 1.