The Call Processing Language (CPL, RFC2824) is an XML-based language, which provides a flexible means of creating end-user services in Internet telephony (VoIP). However, the service description by non-experts does not always guarantee reliable services and, thus, could be a cause of system faults. This paper first proposes six classes of semantic warnings within CPL scripts. For a given CPL script, 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) between multiple CPL scripts, which is a kind of inconsistent conflict between services. We characterize FIs as semantic warnings over multiple CPL scripts. Then, we propose a new FI detection method to combine multiple CPL scripts and to detect script-to-script interactions. We also discuss an architecture to achieve the run-time detection of FIs.