Asterisk

Asterisk is a PBX server program to manage phones.

Extended USE flags
The supports the VOICEMAIL_STORAGE USE_EXPAND variable that is definable in :

Emerge
Install :

Testing
If no PBX hardware phones are available, linphone can be used to test the setup. This is a much less expensive solution than purchasing PBX hardware.

External resources

 * http://www.jdsu.com/productliterature/voipterm-wp-acc-tm-ae-0210.pdf