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T1 · T1-R03

Registration

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Learn it in 5 minutes

  • REGISTER publishes a Contact. It does not place a call.
  • The Request-URI is the registrar domain, not the user's address-of-record.
  • 401 challenges the user. A second REGISTER must use a new CSeq.
  • The binding expires.
  • A registered phone can still be unreachable for media.
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Visual

Registration diagram
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What is going on

RFC 3261 section 10 is the whole idea. CUCM is the registrar for the phones that point at it. A successful 200 means CUCM stored a binding until it expires or the phone removes it. It does not mean the WAN will carry RTP.

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Worked example

First REGISTER, 401, second REGISTER with Authorization and CSeq plus one, then 200. If the second CSeq matches the first, the registrar treats it as a replay.

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Quiz

1. REGISTER creates a voice session.

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Ecosystem

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

  • AOR — The address-of-record being registered.
  • Contact — The device URI to try later.

Config. Compare the phone's CUCM address, the digest user, and the registration timer.

Troubleshooting. A phone that flips between registered and rejected is often a credential, a trust list, or a clock, not a dial plan.

Bridge. MRA still ends in a CUCM registration. The Expressway pair is the path, not a second registrar you invent.

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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