The Analog Display Services Interface (ADSI) was designed to allow telephone companies to deliver enhanced services across analog telephone circuits. In Asterisk, you can use this file to send ADSI commands to compatible telephones. Please note that the phone must be directly connected to a Zapata channel. ADSI messages cannot be sent across a VoIP connection to a remote analog phone.
The res_adsi.so module is required for the Voicemail( ) application; however, the adsi.conf file is not necessarily used. Detailed information about ADSI is not publicly available, and documentation needs to be purchased from Telcordia.