Hello,
Under the user in Web System Administration you could look at 'Voice Mailbox' as this shows the following:
1. Last Web Application Logon
2. Last Phone Logon
Another option would be to go to Web System Administration under 'Monitoring and Reports' then under Reports
You could try running 'User Report' or 'User Message Activity Report' or 'Unused Voice Mail Accounts Report' and see if they might provide the info you are looking.
Please note TAC will no longer provide any support for this product after March 12, 2011
http://www.cisco.com/en/US/partner/prod/collateral/voicesw/ps6789/ps5745/ps6509/prod_end-of-life_notice0900aecd806c3d64.html
An upgrade to Unity Connection 7.x or 8.x should be considered.
Thanks,
Scott