General

CCITT Recommendation E.131 describes, in the form of code schemes, three subscriber control procedures for supplementary telephone services. In order to avoid undesirable proliferation of different types of control procedures, it recommends that Administrations wishing to make supplementary telephone services available to their subscribers should choose one of the three code schemes.

Each of the three code schemes requires the subscriber to send information to the telecommunication system to which he is connected, in a set format and in response to feedback from the system. Certain component parts of the information sent to the system, such as the message suffix, block separators, tone signals and the like, may be considered to be the necessary elements involved in the successful operation of supplementary services.

In order to minimize confusion to foreign visitors, and maximize the benefits that accrue from using elements of known meaning, it is desirable to standardize the usage of elements of codes schemes whenever possible, in particular those elements common to all three code schemes.