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T11 · T11-R01

A SIP trunk is a peer, not a brand

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Learn it in 5 minutes

  • Wikipedia's SIP trunking article describes a SIP service between a provider and a customer's SIP PBX.
  • The same article says the provider side and the enterprise side are different domains, joined by a SIP trunk.
  • SIPconnect, on the SIP Forum page opened here, is a technical recommendation for direct IP peering between a SIP PBX and a VoIP provider.
  • CUCM's trunk, CUBE's dial peers, and the far PBX's own routes are three plans.
  • RFC 3261 is still the request and the response. A trunk does not replace it.
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Visual

A SIP trunk is a peer, not a brand diagram
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What is going on

When CUCM sends a call to Asterisk or FreeSWITCH, count dialogs the same way you count them toward a carrier. CUBE in the middle is a B2BUA, so the Call-ID usually changes. Early offer, PRACK, and session timers are negotiation, not a feature of the logo on the box. SIPconnect is a profile the SIP Forum publishes for provider peering. It is not a promise that every open-source PBX implements it. Read the recommendation if you are matching a provider. Read the far PBX's own dial plan if you are matching a lab peer.

Confirm before relying. Confirm header, cipher, and fax requirements in the SIPconnect version your provider names.

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Worked example

CUCM matches a route pattern and sends INVITE to CUBE. CUBE matches an outbound dial peer and sends a new INVITE to Asterisk. Asterisk's dialplan does not match the digits. The failure is on the third plan. A CUCM DNA trace that stops at the trunk has not seen it.

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Quiz

1. One SIP trunk means both PBXs share one dial plan.

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Ecosystem

Sister pieces sit on the ecosystem page: the glossary, the mind map, the integration map, and the sibling labs.

  • SIP trunk — The signaling peer between two call agents or between a PBX and a provider.
  • SIPconnect — The SIP Forum's technical recommendation for IP peering with a VoIP provider.

Config. Write the CUCM destination, the CUBE dial peer, and the far-side match.

Troubleshooting. If only one direction fails, compare called-number format on that direction before you change codecs.

Bridge. A Webex Calling local gateway is this same shape with a different peer. The Webex guide still wins for that peer.

Mind map, sister technologies, and future hooks live on the ecosystem page.

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Sources

Product screens move. Confirm the guide for your release before you change a live system.

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Sources and go deeper

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

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