Must use class of service "Bulletin board", in my system the cos = 10, but you can use the pulldown menu under edit local subscriber to find yours...
Edit Local Subscriber
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BASIC INFORMATION
* (Required Fields)
*Last Name First Name
*Password *Mailbox Number
*Numeric Address PBX Extension
*Class Of Service 0 - Standarcd 1 - Executive 2 - RVP 3 - class03 4 - class04 5 - Listen Only Greeting 6 - class06 7 - ELA MW 8 - ELA 9 - FAX 10 - Bulletin Board 11 - Transfer Only 12 - Caller App Recording 13 - Plan Admins 14 - class14 15 - NO MWI 16