These and many other defaults can be set in Voicemail Admin. Select Voicemail Admin from the Settings menu and then click the Settings link from the “System View Links” on the settings page.
For purposes of the system, “default” means “the value when you set up the extensions”. What you are asking for is the “gloabl” setting (which is updated for all extensions when you make a change) and there is no Global setting for this option.
Since the extension is already created, the value it had when you set it up would be the same as it was a few minutes ago.