Developing telecommunication applications has evolved from the using closed service architectures as is the case in Intelligent Networks (IN) towards open distributed service architecture such OSA/Parlay, and the Open Mobile Alliance (OMA). The Open Mobile Alliance (OMA) specifies different standardized components called enablers to create an environment in which services could be developed and deployed. Therefore, OMA provides a service-oriented architecture (SOA) and enables the mapping of service elements onto the underlying network infrastructure components. On the other hand, the approach used in MobiLife is more user-centric and tries to bring advances in mobile applications and services within the reach of users in their everyday life by innovating and deploying new applications. Consequently, the MobiLife paradigm also follows a high-level service-oriented approach quite similar to the OMA approach. Issues such as extensibility, scalability, and flexibility are quite critical in determining the success of a certain service. When using the last mentioned approach, service composition on top of OMA enablers or on top of the MobiLife framework is greatly facilitated. The focus of our paper is on the significant differences between the classical black box approach and the modular approach used by the OMA and MobiLife for building telecommunication services on the top of the IP Multimedia Subsystem (IMS) architecture.