There's a lot of assumptions but basically you need:
[ul]
[li]The DID and the associated extension based on your dial plan (1-555-777-1234, Ext. 1234)[/li]
[li]Trunks to Messaging (Assuming SIP) SIP trunk between Communication Manager and Session Manager. You'll also need SIP Entity, Entity Link, and SIP Dial Pattern matching the VM Extension in Session Manager[/li]
[li]Route Pattern in CM to Messaging Trunks[/li]
[li]AAR entry matching the VM extension using the Route Pattern[/li]
[li]Hunt Group (my preference) or UDP entry for the VM Extension to use AAR[/li]
[li]On the 1st page of the Hunt Group, Enter the VM Extension as the Group Extension[/li]
[li]On the 2nd page of the Hunt Group, enter the VM Extension as the Voice Mail Number and Voice Mail Handle. Enter the AAR Access Code (see Feature Access Codes)[/li]
[/ul]