You can set the voicemail greeting of your extension back to default by using an iPhone.
To set your voicemail greeting back to default for your own extension:
1. Launch your App on your iPhone.
2. Tap on the Gear Icon at the bottom left part of the screen to view Settings screen.
3. Tap My Inbound to display My Settings screen.
4. Tap Messages.
5. Tap Greeting.
6. Tap Default to select the default voicemail greeting option.
7. Tap Play to listen to the default voicemail greeting.
8. Review the script of the default voicemail greeting.
9. Tap Save to apply changes.