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T1 · T1-R04

Early media and PRACK

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Learn it in 5 minutes

  • 183 Session Progress can carry SDP before answer.
  • A reliable provisional response wants PRACK.
  • PRACK is its own transaction.
  • Early media is not the same as answer.
  • RFC 3262 is the specification.
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Visual

Early media and PRACK diagram
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What is going on

Carriers often send ringback or an announcement as early media. If the caller hears silence until the 200, the 183 may have been unreliable and ignored, or the early RTP never arrived. Do not mark the call answered because you heard a tone.

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Worked example

INVITE, 183 with Require 100rel and SDP, PRACK, 200 OK of the PRACK, later 200 OK of the INVITE, ACK. The tone is the 183. The answer is the later 200.

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Quiz

1. Hearing ringback proves the called party answered.

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Ecosystem

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

  • PRACK — Acknowledgement of a reliable provisional response.
  • 100rel — The extension that makes selected 1xx responses reliable.

Config. If one side requires 100rel and the other cannot send PRACK, the call fails before answer.

Troubleshooting. Save the 183 and say whether RTP to that address was visible.

Bridge. CUBE can interwork early media between a CUCM trunk and a carrier. Read both legs.

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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