Parlay is an open standard that focuses on opening up the telecommunication networks for new services and new service providers. It encapsulates the underlying protocols and signaling layers and provides a framework for services so that the applications do not need to handle signaling and network related tasks. Parlay is also created to integrate the public telephone network with wireless and packet based networks. Java APIs for Integrated Networks (JAIN) is the Java implementation of the Parlay specification and includes a Java framework for network access and call control, among others. I have in this thesis used the Parlay and JAIN technology to design a Virtual Private Branch Exchange (VPBX), which is a software based system for the telecommunications domain. The VPBX provides functionality such as attendant control, routing and re-routing of calls, telephone queue handling and calls on hold. All use of the system is made from mobile phones, even the attendant client application is designed for a handheld device. I have in this thesis presented an architectural design for the VPBX and an evaluation of the design.