 the sub- scriber mobile number (i.e., of Linda) to issue real-time charging for this call. The charging model, in turn, would depend on the subscription plan of Linda (prepaid, postpaid, premium, etc.) and hence it is typically a business decision taken and programmed into the SCL by business experts. Of particular interest, for example, is the scenario where Linda does not have an account with the operator. In this case, SCL pushes a SIP message to Linda's device-the message includes an HTTP URI which Linda can visit to buy a "visitor pass" with some credits in it (Figure 1.8b). Subsequently, SCL sets up a visitor account for Linda at runtime using the capabili- ties of the charging platform. Linda can now proceed to make the call and talk to the cabbie-to decide a time and location for the pickup. Once again, SCL (in a B2BUA mode) controls the entire call session and applies real-time charging to the call. Finally, when the call terminates (indicated by a SIP BYE), SCL pushes a sec- ond message to Linda's device. The message includes an HTTP URI which Linda can visit to confirm her appointment with the cab driver. SCL charges Linda and the cab driver based on the charging policies of Business Finder. Finally, Business Finder updates Chris's presence status to show that he is now busy picking up a passenger. Service Composition and Control 1.5 Discussion A generic model of Telecom services is described in [2], where apart from core Telecom network functionality and third-party offerings, services also make use of device functionality and information such as calendar, user profile, and location (e.g., cell site information). Currently, there are a plethora of end-user devices in a Telecom environment, ranging from basic (offering only the capabilities of messag- ing and voice) to sophisticated, state-of-the-art devices such as the iPhone.* There are efforts to promote an open mobile phone software stack that abstracts hardware differences and offer a uniform set of APIs for accessing many of the functions of a m