The Call Processing Language (CPL) is one of the best known approaches developed for creating telephony services in Internet telephony. Its XML-based structure makes it a flexible approach to create services not only by service providers, but also by end-users, which increases the possibility of feature interactions. In this paper we present an online approach for detecting feature interactions of CPL. A feature interaction manager is introduced to handle the problem. The manager detects feature interaction through comparing actions indicated by real-time SIP messages and actions indicated by CPL scripts which we call intentions. We evaluated the proposed approach using the VOCAL (Vovida Open Communication Application Library) VoIP system. Among the ready-made features, a feature interaction has been detected between Call Screening and Call Forward All Calls.