Try this: (Taken directly from the Partner ACS Installation Guide)
Programming For VMS Cover Rings
To change the number of times calls ring before being sent to the voice messaging system:
1. Press FEATURE 00 SYSTEM PROGRAM SYSTEM PROGRAM #321 at extension 10 or 11.
2. At the extension prompt, enter an extension number.
(The Cover Rings prompt appears: "VMS Cover Rings XX
3 Rings")
3. Enter a setting (1–9; 3 is the factory setting), or press D until the correct value appears:
If it ain't broke, I haven't fixed it yet.