When you delete a voice message, it automatically goes into a Trash folder. Leaving clutter in your trash means you have less storage on your voicemail server and may prevent people from being able to leave you a message.
Messages remain in your Trash folder unless you:
Recover a message. Then, the message moves back to your Inbox.
Empty your trash.
Leave your messages in your trash for a certain length of time and they're automatically removed. Your administrator determines how long messages stay in your trash.
Go to your Voicemail , click the Trash tab, and then select Empty Trash.