This article has double significance. It aids in understanding how to build call control applications using Parlay. It also gives insights into how to build gateways that support mapping onto SIP. The authors give an overview of the Parlay call control APIs. The mapping issues are then dealt with. The article next describes the implementation of the gateway. After that the authors discuss related work and lessons learned. The conclusion provides a summary and identifies areas for further work. Two main sets of standards have emerged for Internet telephony: H.323 from the International Telecommunication Union--Telecommunication Standardization Sector and session initiation protocol (SIP) from the Internet Engineering Task Force. Parlay, a set of standard object-oriented and signaling protocol-neutral application programming interfaces (APIs), is an alternative. Call control makes it possible to establish, modify, and tear down calls. It is the main functionality offered by Internet telephony networks. The authors have built a call control application in a SIP environment, using the call control APIs offered by Parlay. This article describes the case study. The mapping of the APIs onto SIP is presented, and its implementation is described. Related work is reviewed, and the lessons learned are discussed. Parlay call control APIs are suitable for application development in Internet telephony. However, well isolated extensions are needed to realize their full potential