Brad Porter. <a href="http://www.w3.org/TR/2001/WD-call-control-reqs-20010413"><cite>Call Control Requirements in a Voice Browser Framework.</cite></a> 13 April 2001. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/2001/WD-call-control-reqs-20010413">http://www.w3.org/TR/2001/WD-call-control-reqs-20010413</a>