Door Entry Telephone Systems can be connected directly to a telephone network. The system provides a simple and secure solution for answering and opening remote doors via a conventional telephone handset operating over BT or Cable networks. The system is suitable for both commercial and residential buildings Especially buildings with multiple tenants. Because of buildings like this, installation wiring of a conventional system would be difficult and expensive. Installations can also be integrated with your existing telephone system by calling the office telephone extension number or by calling a designated office telephone.