Many open source hackers see the allure of building applications that integrate with the telephone network. However layers of industry jargon, proprietary technology and arcane knowledge lead many to believe that telephony is "hard." Adhearsion will change their minds. The Adhearsion project is an open source voice application development framework, the first of its kind in the open source world. In this talk we will build several working voice applications and demonstrate how to integrate them with existing PBX installations. Along the way, we will discuss voice application design and show how to avoid common pitfalls.