Trunk · early offer
CUCM to the PSTN
A national call is three signaling hops and two media negotiations.
- INVITE — The phone's CSS finds the route pattern.
- INVITE — CUCM may send a delayed offer toward CUBE.
- INVITE — CUBE offers early toward the carrier when that dial-peer requires it.
- 180 — Alerting from the public network.
- 200 — Carrier SDP answer.
- 200 — If this 200 has no SDP, CUCM never receives the answer. That is a classic one-way call.
- RTP — Media is between the negotiated addresses, not along the SIP hops.
Trace both CUBE legs. Compare the SIP profile on the inside dial-peer before you open a carrier ticket. Signaling port 5060 or 5061 is not the RTP range.