Recommendation I.251.1 (11/1988): Number identification supplementary services: Direct-Dialling-In (DDI)
superseded recommendation/international standard from the International Telecommunication Union
ITU Telecommunication Standardization Sector
Number identification supplementary services: Direct-Dialling-In (DDI)
Recommendation I.130 (11/1988): Method for the characterization of telecommunication services supported by an ISDN and network capabilities of an ISDN
Recommendation I.140 (11/1988): Attribute technique for the characterization of telecommunication services supported by an ISDN and network capabilities of an ISDN
Recommendation I.210 (11/1988): Principles of telecommunication services supported by an ISDN and the means to describe them