If the lines are in pools, which is typical for most Legend systems, you can't assign principal owner for the lines. Avaya recommends using a calling group with no members, and using time-based overflow to send calls to the voice mail calling group. In the Voice Mail, you need to create a mailbox with the same extension number of the delayed calling group, and assign it as an auto-attendant (done through class of service setting on the older Merlin Mails).
Here is a cut-n-paste of a job aid that I use when initially setting up Merlin Messaging, that should apply to the Merlin Mail also:
Set for Delayed Call Handling
Assign Lines/Pools to Delayed Call Handling Group
F1 - SysProgram
F5 - Start
F6 - Extensions
PgUp - More
F4 - Grp Calling
F10 - Line/Pool
791 - Enter extension number of delayed call handling group
F10 - Enter
nn or nnn - Enter Pool or Line Number
F10 - Enter
F5 - Exit
F3 - GrpCoverage
770 - Enter extension number of Merlin Messaging calling group
F10 - Enter
30 - Enter Coverage Group Number
F10 - Enter
F5 - Back
F5 - Back
F4 - Grp Calling
F8 - Overflow
791 - Enter extension number of delayed call handling group
F10 - Enter
770 - Enter extension number of Merlin Messaging calling group
F10 - Enter
F1 - Number Based Overflow
Alt-P - Erase Current Setting
99 - Enter the Highest Setting
F10 - Enter
F2 - Time Based Overflow
Alt-P - Erase Current Setting
nnn - Enter number of seconds before Automated Attendant should answer - 5 sec/ring cycle
F10 - Enter
I dont have a manual on MerMail nearby, so I don't have the correct COS for the AA mailbox.