protocol to the subscriber tlna by a set up ack message the line unit 12 then begins searching for a peer line unit within the switching center 10 for this purpose it sets up a signaling connection to the central processor 26 with the aid of internal signaling messages the line unit 12 transmits the call number of the subscriber tlnb to the central processor 26 the central processor determines a peer line unit on the basis of the call number in the exemplary embodiment the line unit 14 the line unit 12 is notified of this via internal signaling messages the line unit 12 subsequently sets up a signaling connection 46 to the line unit 14 and passes on the received set up message to the line unit 14 with the aid of an internal signaling message the line unit 14 then signals to the terminal device of the subscriber tlnb a set up message in accordance with the standard dss1 if on the other hand the subscriber tlna wants to reach a subscriber who is connected to a switching center other than the switching center 10 the central processor 26 determines the line unit 18 as the peer line unit in this case an internal signaling connection 48 is set up between the line unit 12 and the line unit 18 the line unit 20 serves for the conversion between the isup protocol and the internal protocol the conversion takes place in a trunk unit 50 the.