Abstract: A circuit arrangement sets up conference calls in a digital time-division multiplex telecommunications switching center. For setting up three-party connections, a conference unit (KE) having a PROM is provided wherein are stored all possible, companded sum and difference values of individual contributions of respectively two conference participants. These are selected by the individual contributions as read selection addresses. The PROM thus realizes the functions of linearization of the sum and difference and of companding that are performed by separate sub-units in previous conference units.

Abstract: In a method for monitoring existing telephone connections, particularly permanently switched telephone connections, the information that correspond to one another and come from the redundant switching network halves (SN0, SN1 ) are compared to one another. The information intended for forwarding to connected lines is branched in the interface (LTU) at the line side into a separate time channel (K0) and is transmitted back to the interface (LIU) at the switching network side and are compared thereat to said information. Information coming from the lines is branched into the separate time channel and is transmitted in parallel with the original information to the interface (LIU) at the switching network side. The branched information is compared to the original information in the interface (LIV) at the switching network side. The procedures ensue time-channel-by-time-channel.