1. Mobile cellular and Internet user penetration worldwide, ITU 1997-2007 ICT Market Information and Statistics, http://www.itu.int/ITU-D/ict/statistics/maps.html
2. Roe, D.B., Wilpon, J.G. (eds.): Voice Communication Between Humans and Machines. The National Academies Press, Washington (1994)
3. Voice Extensible Markup Language (VoiceXML) Version 2.0, W3C Recommendation (March 16, 2004), http://www.w3.org/TR/voicexml20/
4. Voice Extensible Markup Language (VoiceXML) 2.1, W3C Recommendation (June 19, 2007), http://www.w3.org/TR/voicexml21/
5. VoiceXML Forum, http://www.voicexml.org/