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• ITU-T RECOMMENDATION I.251.1 (1988)
• 1 I.251.1 - Direct-Dialling-In
  • 1.1 Definition
  • 1.2 Description
  • 1.3 Procedures
  • 1.4 Network capabilities for charging
  • 1.5 Interworking requirements
  • 1.6 Interaction with other supplementary services
  • 1.7 Dynamic description