IP Multimedia Subsystem (IMS) is a standardized service control overlay meant to offer rich IP multimedia services to the users in different networks. However, IMS service invocation as specified by 3GPP encounters multiple issues and ambiguities including "service conflict" and "service composition" management. In this paper, we highlight these issues by focusing on the following question: How should IMS interact with multiple services during a single IP multimedia session while 1) detecting and resolving potential conflicts between these services and 2) ensuring the interoperability and cooperation between these services? Based on the requirements and characteristics of Next Generation Network (NGN) service architecture, we propose architectural and functional improvements to the service invocation mechanism of IMS in order to answer to this question.