On the wear-hard mailing list, uberborg Doug Sutherland mentions

Developing Voice-only Applications in the Absence of Speech Recognition Technology http://www.cc.gatech.edu/fce/savoir/pubs/savoir.html

and

Caiviar is an Interactive Voice Response (IVR) System based on CAPI. It runs on Linux and Windows systems, and supports Text-To-Speech (most notably Realspeak and Festival), multithreading, Barge In, 3 Party Conference, G3 Fax etc. It has a very simple interface, allowing fast creation of Telephony applications, like e.g. Voiceboxes, Authentication systems, automatic Callcenters etc. Languages supported include Java, C++ and Perl. The source code for Caiviar is freely available.

http://www.quiss.org/caiviar/

TTS+Caiviar HOWTO http://www.quiss.org/caiviar/TTS-Caiviar-HOWTO