Hi Deepak,
Have you tried this command?
Unlocking a Voice Mailbox
If a mailbox becomes locked, the telephone subscriber will hear a message stating that the mailbox is unavailable. Use the voice mailbox unlock command to unlock the mailbox.
Starting in Cisco Unity Express EXEC mode, enter the following command:
voice mailbox unlock {owner name | telephonenumber tel-number}
name Name of the mailbox owner.
tel-number Extension or telephone number of the mailbox.
Example:
se-10-0-0-0# voice mailbox unlock owner user3
se-10-0-0-0# voice mailbox unlock telephonenumber 50174
From this cli guide;
http://www.cisco.com/en/US/products/sw/voicesw/ps5520/products_administration_guide_chapter09186a00806f11ef.html#wp1060725
Hope this helps!
Rob