The programmable service for Internet telephony (VoIP) allows end-users or third parties to define their own customized services. However, it imposes a serious drawback that service description created by end-users is likely to contain problems that are semantically ambiguous or inconsistent. To cope with this problem, we have so far proposed semantic warnings, which are the guidelines to guarantee the semantic correctness for the CPL (Call Processing Language) programmable service environment. In this paper, we evaluate the proposed semantic warnings with practical VoIP system, VOCAL (Vovida Open Communication Application Library). In the experiment, the proposed warnings revealed a semantic redundancy in a ready-made feature of VOCAL. It is also shown that customized features containing the semantic warnings often led VOCAL to problematic situations. Thus, the proposed warnings can help feature provisioning system to detect semantic flaws in programmable service environment.