Hi Rajesh,
Just to add a note to the great tips from Tim (+5 points Tim:)
You really only have two choices here that I can think of (without sharing the Password)
If you were running Unity 5.x (which I know you aren't, but for future reference)you can config Unity to use the "Last Redirecting Number" on a Call Forward and have your user Forward to another person who is covering for them while they are away. This way the Messages will be left in the "covering" persons mailbox.
But in your case, if you don't want to have to re-config for each user Holiday I would encourage your users to record the "Alternate greeting" during a leave of absence that references callers to re-direct their call to the applicable number.
You can customize how Cisco Unity handles calls to subscribers who have the alternate greeting enabled. For example, you can specify that for as long as the alternate greeting is enabled, Cisco Unity:
•Transfers callers directly to the greeting without ringing the subscriber phone when calls are transferred from the automated attendant or a directory handler to the subscriber extension. (The subscriber phone will ring if an outsider caller or another subscriber dials a subscriber extension directly.) This option is particularly well-received by subscribers who share a phone.
•Prevent all callers from skipping the greeting. In this way, you can increase caller awareness of a subscriber absence.
•Prevent all callers from leaving messages. By specifying that Cisco Unity will prevent all callers from leaving messages, you can help reduce mailbox size when a subscriber is out of the office and does not plan to check messages regularly.
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/405/administration/guide/ex/sag_0170.html#wp1149748
For Unity 4.x Versions;
For this configure a Voicemail profile (in CCM) for User B and apply it to User A that will allow this type of Call Forward to User B's mailbox.
Or in Unity set up Alternate Extensions so that User A is an Alternate Extension for User B etc. Sharing a Cisco Unity Voice Mail Box between Two or More IP Phones
Configure Alternate Extensions
Open the Unity System Administrator web page.
Navigate to the subscriber's profile. Select Subscribers > Find and Select a Subscriber > Enter Subscriber Information then click Find and click the Subscriber's name for the subscriber that owns the primary phone.
When the subscriber page comes up, select the Alternate Extensions option and click Add.
Enter the alternate extension number and click the Save icon.
From this good Unity doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_configuration_example09186a008015ceec.shtml#steps
Hope this helps!
Rob
