The Call Processing Language (CPL in short, RFC2824) is an XML-based language, which provides a flexible mean to create end-user services in the Internet Telephony (VoIP). However, the service description by non-experts does not always guarantee reliable services, thus, could be a cause of system faults. This paper first proposes six classes of the semantic warnings within the CPL scripts. For a given CPL script, the semantic warnings identify the sources of ambiguities, redundancies and inconsistencies, even though the script is syntactically well-formed with respect to the Document Type Definition. Next, we address the problem of Feature Interaction (FI, in short) between multiple CPL scripts, which is a kind of inconsistent conflict between services. We characterize the FIs as the semantic warnings over the multiple CPL scripts. Then, we propose a new FI detection method to combine the multiple CPL scripts and to detect script-to-script interactions. We also discuss architectur...