The "Voicemail Admin Module" allows you to make changes to the default voicemail settings used by the system, and to make changes to voicemail settings for each individual user.
It also allows you to view statistics about the voicemail system on a global or per user basis, and to delete files relating to voicemail on a global or per user basis.
It also allows you to define timezone definitions for use by the voicemail system.
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.