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The "Voicemail Admin Module" is related to the Extensions Module, because it sets certain defaults that are used in the Voicemail section of the Extensions Module Module, and because it allows you to set individual settings and view individual statistics for extensions that have been defined on your system in the Extensions Module Module. If you make a change to a setting for a particular extension in this module, that change will appear in the Voicemail section of the Extensions Module module for that extension, either in the pre-set fields or in the "VM Options" field.
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From the top menu click Settings
From the drop down click Voicemail Admin