Hi there,
Your requirements are very vague which introduces some risk into this project, hence the higher price. I'm happy to revise my offer upon receiving more detail.
I have extensive experience in both Asterisk and WebRTC from a telephony perspective and extensive experience in Web Development. I have built several WebRTC projects using HTML5, JavaScript, Python, PHP, Meteor and WebSocket.
I previously worked for several carriers with the most notable being the Skype engineering teams at Microsoft responsible for all call signalling.
So in a nutshell, I understand VoIP backwards!
Looking forward to working with you!
Regards, Justin