If it's nupoint voicemail, then this is working as you would expect.
From the nupoint documentation:
FCOS bit 023 Make and Mark Confidential
[highlight #FCE94F]Allows the user to make a message that cannot be given by the recipient to another user[/highlight]. The user presses M to make a message, and records a message. To mark the message confidential, the user presses M to reach the Message Addressing menu, then presses C for confidential. (To cancel confidential, the user presses C again.) To send the message, the user presses X until the server announces that the message has been sent.
Requires feature bits:
•020 (Make messages)
•040 (Receive messages from other users) in the recipient mailbox