Pick an extension number on your system, at the top end of the dial plan that is not already in use.
Partner Plus, that's 10-33
Partner II, 10-57
Older ACS, 10-49
Later ACS, 10-57
In the Partner, assign Auto VMS Coverage, #310, and if you can, set Auto VMS Coverage Rings in #321 to 1
In the Partner Mail, create a matching mailbox, with a Class of Service of 15, to steer it to AA-1
Now transfer the caller to that extension, and hang on the make their selection for them, then hang up to connect them.