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Collaboration, Voice & AV · Foundational

SIP User Agent (UA)

A SIP endpoint that originates requests as a user agent client (UAC) and answers them as a user agent server (UAS).

Definition

Every SIP device that creates or receives sessions is a user agent. The UAC role sends requests such as INVITE, and the UAS role generates responses such as 180 Ringing and 200 OK; a phone switches roles depending on whether it is calling or being called. IP phones, soft clients, gateways and voicemail servers all contain user agents.

Example

A Webex App client acts as a UAC when it sends INVITE to CUCM and as a UAS when it receives an incoming call.

Exam tip

UAC and UAS are roles per transaction, not device types.

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