The ClicK-to-Dial application is an example of a converged application with UAC/S behaviour.
The user of the application uses a web page or a web service to enter the callee adress as well as the details (username,password,domain) of the caller SIP account that will be used to make the calls. With all this data the application starts a 3pcc (3rd party call control, see RFC 3725) that first contacts the caller. When the caller answers a second callee call leg is started. When the callee answers both call legs are bridged using standard 3pcc techniques. The client (user of the web page or programatic webservice client) asks for the state of bot call legs using HTTP or SOAP calls repeatedly.