You first refer the information on SIP and H.323 standards.
Then visit the sites paltalk.com,phonefree.com, pingtel.com and IPstarphone.com. like this u have thousand of sites. Apart from this you first visit voip-calculator.com for better idea on your project. I am also working on the same line, if you require some more information please get in touch with email@example.com.
I think this is sufficient information to start your work.
Best of luck.