This document is about the development of a modular entry phone terminal which connects to the public switched telephone network. The rapid advancement of the microelectronics and the telecommunication has also made an impact on the entry phone devices that give security and access services in a way that new ideas and solutions were taken into the new products. The entry phone connecting to the public telephone network is also a part of those new solutions that differ from the old one by using an already existing telecommunication network. Nowadays almost all the buildings have a public telephone network and beside that they also have an other communication network used by the entry phone system. But this new solution integrates the telephone network of the entry phone system into the public telephone network and this way a less amount of wires have to be used in the walls of the building. The development was directed to the design of the controlling unit realized by a microcontroller circuit whose duty is to operate all the entry phone terminal setups that can be built up with the different modules. In the course of the development the services of the recently available entry phone systems and the important requirements for connecting to the public telephone network were taken into account. The development started with a system design that was used in the process of the circuit design and in the programming of the controlling unit.