It is known practice in modern communication networks to observe, by way of example, the network state of other parties in the same communication installation.  Hence, busy lamp arrays or PC applications are used at the switchboard positions of the communication installations to indicate whether each party is currently free or busy or is in the call state. 
In addition, terminals in communication installations are frequently equipped with direct call keys which can be used to call another party in the same communication installation by pressing a key.  These direct call keys usually have an associated LED or other indicator element, with the network state of the associated party being indicated by the LED lighting or flashing, for example. 
If a plurality of PC applications are intended to be operated in one communication installation, then the information about the network state of the visually indicated internal parties needs to be buffer-stored on a server and forwarded from there to the PCs to indicate the state.  To this end, each monitoring PC application stores on the server the telephone numbers as addresses for the terminals which it is to monitor.  The server then uses a data link to prompt the start of a monitoring process in the communication installation.  This monitoring instruction is frequently also called “setting a monitoring point”.  In the event of a change of state in the monitored component, the communication installation sends a corresponding message to the server, which forwards it to the PC applications.  A server may also collect and forward state information about the internal parties in a plurality of communication installations in a communication network in the manner of a network complex, by virtue of a respective data link being set up to each communication installation. 
A drawback which has been found with the various arrangements is that the services for monitoring party states in a communication network are respectively limited to the parties in an individual communication installation or to the parties in a few communication installations combined into a network complex.  Parties which are not connected to the same communication installation or to the same network complex, and also the service, cannot be monitored.  In addition, connecting a plurality of appliances for the purpose of visually indicating party states and for the purpose of monitoring parties in a plurality of communication installations requires a central entity in the communication network, that is to say a server.