Telephone systems form an interface between the public- to the internal building telephone network. Modern telephone systems enable an IP-based and tap-proofed communication. Thereby, flexibility of the systems is very relevant. Implementation of a fax-service here is a criterion which should not be ignored.
During planning telephone systems, necessary interfaces have to be defined. Integration of intrusion detection-, intercom-, clock-, fault alarm systems, transmission of vision and sound and, not least, information technology is possible at many systems.