Lucent Technologies Octel Unified Messenger Concepts and Planning Guide 101-1620-005 - Manual (Page 39)

Sizing the system
101-1620-005 2-13
Disk space
requirements Voice messages require 4 Kbytes per second (240 Kbytes per minute) of
space for storage.
Subscriber mailboxes
A subscriber who receives five voice messages per day, with an average
message length of 60 seconds, requires 1.2 Mbyte of storage space to
keep all messages stored on the Exchange server or on another machine if
the subscriber stores the messages in Personal Folders. However, most
people have no need to store all of the messages they receive. Subscribers
should be encouraged to remove unnecessary copies of messages from
their Exchange folders. For example, they should make sure the Deleted
Items and Sent Items folders are not retaining copies of messages longer
than necessary.
Off-line message taking
When the Exchange server is not running, all voice messages from
outside callers are temporarily stored on the Octel Unified Messenger
voice server. This offline spooling facility requires enough available disk
space on the voice server to hold all voice mail coming in during that
time.