The Duo system allows various advantages for both the installer and the end user. First of all, it satisfactorily responds to the request to avoid masonry work on the wall for the external unit. It also allows simple wiring of only 2 non-polarized cables where previously there were 5 and did not guarantee the performance of now. It is also important to allow integration to future services for any next new requests and Duo System allows all this (for example through a possible gateway module for video call service on smartphones even for just one user or a few). Furthermore, it offers the end user ease of use both in the internal call phase through the display module with automatic search keys, and the possibility of writing a courtesy message on the display module (here the data of the condominium administrator) as well as the security of access control via digital keypad module.