Asterisk Voicemail for iPhone
allows you to check your voicemail messages on your Asterisk phone system from your iPhone.
It works similar to the native iPhone Visual Voicemail, allowing you to list messages, listen to messages, display caller-id information, delete messages, move messages, return calls and change voicemail settings all from your iPhone.
The technology behind it is Asterisk
(The Open-Source VoIP PBX), PHP
for the backend, Smarty
for the frontend.
Also, check out my MythTV for iPhone