Unfortunately, once a voicemail message has been deleted there is no way to retrieve it. At the end of the below mentioned expiry periods, voicemail messages will be purged from the network and will no longer be accessible.
Follow the instructions and press that number. You will be asked to delete your current Voicemail greeting. Click on the ‘Yes’ button that appears on the screen if you want to remove that. You can save your new recorded Voicemail greeting or can use the default recorded option. From the home screen, select the Phone app. Note: Alternately, you can set up voicemail by selecting the Phone app then selecting and holding the 1 key. Select Keypad, then select the Visual Voicemail icon. Select Start. Select Ok. From any Home screen, tap the Menu key. Tap Settings. Tap Call settings. Tap Voicemail. Tap Voicemail again. View the voicemail number. Press and hold the 1 on your device’s dial pad to call and connect to your voicemail. Press the star * key to access the main menu. Press 4 to access the mailbox options. Press 7 to change the language of the voicemail prompts to either Spanish or English.

When you know something to be true, when you have an opinion that needs to be shared, be more assertive and powerful in your word choices. Here are 3 simple ways you can do that.
Website: https://www.bemidjistate.edu/offices/its/knowledge-base/how-to-change-your-voice-mail-greeting-on-your-mitel-phone/