In ISDN Q.931 / Q.850 signalling, Cause Number 1 is defined as “unallocated (unassigned) number.”

The network switch cannot find any endpoint or directory entry that owns the dialed DNIS, so it immediately releases the call with cause 1. This exactly matches option D: “The dialed number is not assigned to an endpoint.” Media resources, user-busy, or call-rejection conditions use different cause numbers (16-series or 20-series).

A. Media resource problems occur after call setup; no ISDN cause code corresponds—cause 1 is issued before media allocation.

B. “Destination rejects the call” maps to Cause 21 (Call rejected), not 1.

C. “Destination number is busy” maps to Cause 17 (User busy), not 1.

1. ITU-T Recommendation Q.850 (02/1998), §3.2, Table 1 – Cause 1: “Unallocated (unassigned) number.”

2. Cisco Systems, “Understanding and Troubleshooting ISDN Call Disconnect Problems,” Doc ID 10036, Table “Cause Codes,” cause 1 – Unallocated/unassigned number.

3. Cisco Press, Caslow et al., “Cisco Voice Gateways and Gatekeepers,” 2001, Ch. 3 “ISDN Call Control,” p. 83 – list of Q.931/Q.850 cause codes shows cause 1 as unallocated number.