Phase 1 - sip can be integrated to open vpn.
Phase 2- Voip (soft client) can also be integrated with web application.
I have only one doubt- Are you making a web-conferencing software?
Profile- Profile: We are techvisakha. Mr. Sesha has more than 14 years of experience in handling large IT projects spanning multiple Industry verticals. Our team has nearly 4 years of experience in building windows and web applications using various technologies like Java, Javascript, HTML5, CSS3, j2ee, spring framework, hibernate,jquery, etc..