Cheat code

Troubleshooting tree

Registered? If no, DNS, TFTP or edge, 401, time, certificate.
Inside calls work and outside reorder? CSS, then route list, then an empty CUBE trace.
Connected and one-way? SDP on both answers, then RTP both ways, then QoS.
Reserved and silent? The desktop state is not the ring. Capture the phone leg.

Source: RFC 3261 for the signaling checks. The 29 cases are the longer trees.

Sources and go deeper

Opened 2026-10. A row marked confirm has no link, because that page was not verified.

RFC. RFC 3261, SIP last checked 2026-10
RFC. RFC 3264, offer/answer last checked 2026-10
RFC. RFC 3550, RTP last checked 2026-10
Cisco. Cisco Unified Border Element product support last checked 2026-10
Cisco. Understand IOS and IOS XE call routing last checked 2026-10
Cisco. System Configuration Guide for CUCM, Release 15 last checked 2026-10
Protocol and ports. CUCM 15 TCP and UDP port usage last checked 2026-10
Community. Cisco Community, Collaboration knowledge base last checked 2026-10
Wikipedia. Session Initiation Protocol (Wikipedia) last checked 2026-10
Wikipedia. Real-time Transport Protocol (Wikipedia) last checked 2026-10