A more technical approach of writing VoIP Java applications for OpenSER and WeSIP was given by Elias Baixas of Voztelecom. He went through SIP servlet concept and how its API eases the development of VoIP application without any need of technical details about SIP/VoIP internals.
With about two slides of Java code you can embed in your web application a VoIP click to dial feature. Also, as showed in the slides, WeSIP provides seamless interoperation of SIP and HTTP Servlets. web developers will find lot of similarities that will make VoIP SIP Servlets something easy to do if you have HTTP Servlet knowledge.
Download slides of Development of convergent J2EE applications for OpenSER