Employers are generally permitted to access their employee's voice mail and e-mail messages. The key is to establish and communicate a policy that eliminates any reasonable expectation that e-mail and voice mail will be private.
Listening to or recording confidential communications is prohibited if either party to the conversation has a reasonable expectation that it will be private.
When in doubt, always consult with legal counsel.
You can find more information in the Monitoring Employees section of the HR Library.