 supplementary services include the following: Call forwarding permits you to direct incoming calls to another tele- phone. Forwarded calls are regarded as being made from your home telephone and will therefore be charged to the telephone bill of the subscriber who has forwarded the call. Introduction to Telecommunications Network Engineering Call waiting means that, during a call in progress, a subscriber is notified of an incoming call. You hear the message as a faint tone in the receiver, while the caller simultaneously hears a normal ringing tone. You can alternate between these two calls. Automatic callback can be used when the number you are trying to call is busy. A subscriber notifies the system that you want to have a call established when the called party becomes free and she will be informed when this happens. When the subscriber then lifts the receiver, the number will be automatically dialed again. Abbreviated dialing permits a subscriber to specify short numbers that correspond to complete telephone numbers you use most fre- quently. These short numbers can be used by all home telephones that are connected to the same subscriber loop. Screening of incoming and outgoing calls allows a subscriber to specify which telephone numbers he does not want to receive calls from or make calls to. This service is implemented by the telephone service provider according to a customer request. A subscriber may, with the help of this service, avoid charges that may be very high when expen- sive service numbers are called from his telephone. Implementation of supplementary services in local exchanges is reason- able because these services are related to only one subscriber connected to one exchange. A subscriber is also able to modify the service and there is no need to transfer service information to other exchanges. However, some services should be available in all exchanges. Examples of this include use of the same emer- gency number all over the country and establishment of nationwide service numbers. Calls to th