INTERNATIONAL TELECOMMUNICATION UNION ITU-T RECOMMENDATION SUMMARY Rec. No. : Q.951.1 Title : Stage 3 description for number identification supplementary services using DSS 1 Study Group : XI - Switching and Signalling Version : New Date of adoption : 1992 Notes : DIRECT-DIALLING-IN (DDI) This service enables a user to call directly to another user on an ISDN private branch exchange (ISPBX) or other private systems, without attendant intervention. This supplementary service is based on the use of the ISDN number and does not include Sub-addressing. DDI numbers form part of the international numbering plan according to Recommendation E.164. Recommendation Q.951.1 provides the flexibility to Administrations to use national numbering plans of fixed or variable lengths. This flexibility also applies to DDI numbers, i.e. even within a given ISPBX or other private system, DDI numbers of different lengths may appear. In networks with an open numbering plan, the length of the DDI number is not necessarily known by the servicing local exchange nor by any other entity of the public network. DDI is provided when at least a part of the ISDN number which is significant to the called user is passed to that user. These last digits (fixed or variable length) are sent from the exchange and received by the ISPBX or other private system by en-bloc or overlap receiving, which finally and automatically establish a call to the destination without the assistance of an operator. These last digits are sent in accordance with Recommendation Q.931, § 5.2. To order the complete text of this Recommendation, please use the Order Form for ITU-T Recommendations. An electronic version of this form is available on ITUDOC (Winword 2.0: UPI=ITU-5265; ASCII: UPI=ITU-2488).