Correct Approach and Requirement

I am working on public facing web application and need to build Video Conference for the end user. Although i am experienced Application developer on Microsoft Stack but have very light knowledge about SIP protocols.

I am looking for experts like you to guide me or point me to right direction so i can start build the application.

I am more interested in Open Source api for development of VoIP and Video Conference application.

Possible please put me in the right direction how shall i start using FreePBX for
to develop the application.

thanks !!!

FreePBX is a web front end for Asterisk, it provides no SIP services.

Asterisk 10, in beta, provides Video MCU functionality.