Asterisk is a complete PBX in software, and is the execution platform for all telephony applications developed with DialogPalette.
Asterisk - The Open Source PBX

DialogPalette currently makes use of the following text-to-speech technologies:
Festival Speech Synthesis System
Flite Speech Synthesis Engine

The Asterisk "Flite" application was developed specifically to function with an extended version of the Flite TTS engine, developed by the HLT Research Group at the Meraka Institute, and the University of Pretoria. This work is available on as the "TTS-Cubed" project.
Human Language Technologies - Meraka Institute
University of Pretoria