Method for manual or automatic connection of an ancillary device to digital telephone stations

The invention relates to a method for manual or automatic connection of an ancillary device to digital telephone stations in digital telephone systems. The stations have one or more dial-up lines to the telephone system comprising two signalling wires and two speech wires. The telephone system must be able to identify whether a dial-up line has been seized by an ancillary device or by the telephone station and when the dial-up line is to be released. This object is intended to be achieved with the simplest means and without expensive line indicators. Furthermore, simple call monitoring is intended to be provided for automatic connection of the ancillary device. To achieve this, criteria for automatic connection and criteria for monitoring an incoming call are provided in digital memories in the telephone system, and criteria for line monitoring and seizure definition are provided in a supplementary module. The method is suitable for key telephone systems (series telephones, intercom systems) and extentive-secretary systems.