The easiest way to build a browser based phone is to use the free library called SIPjs.
You can download it from here: http://sipjs.com/download/
These articles will be based on Version 0.7.5.
Once you have downloaded the file, you should also include jQuery, to assist will building the page.
You can download it from https://code.jquery.com/jquery/
These articles will be based on Version 2.2.3 (although just about any version will do)
Once you have downloaded these libraries, make your phone page by starting a blank html page as phone.html and save this file to a folder location together with the jquery-2.2.3.min.js and sip-0.7.5.min.js files.
Please note: You cannot use your computers microphone (getUserMedia) if you try to access this page without an SSL certificate. The best option is to enable the self signed certificate on your local IIS server and demo the page using https://localhost/page.html