This standard defines the Keypad signalling protocol for the purpose of supplementary service control at an interface at the S reference point between a Terminal Equipment (TE) and a Private Telecommunication Network (PTN). A PTN consists of one or more interconnected Private Telecommunication Network Exchanges (PTNX)@ and therefore an interface at the S reference point is actually between a TE and a PTNX. The S reference point is defined in Standard ECMA-133. The Keypad protocol operates in conjunction with the signalling protocol specified ECMA-106. It is based on use of the Keypad facility and Display information elements. While the generic procedures associated with keypad invocation are specified in this standard@ the allocation of the access codes used to request or indicate a supplementary service are not standardized. This Standard is applicable to TEs which are intended for connection to PTNs@ and is also applicable to the user accesses of PTNs.