User Agent
In SIP, a user agent is any endpoint that originates or receives requests: a softphone, desk phone or gateway. It acts as client and server at the same time.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
A user agent is a SIP endpoint that both sends requests (user agent client) and receives them (user agent server), such as a softphone, desk phone or media gateway.
A user agent, often shortened to UA, is the SIP term for an endpoint. Softphones, desk phones, analogue telephone adapters and PBX trunk interfaces are all user agents. RFC 3261 section 8 defines how they behave.
Each user agent has two roles. When it sends a request, such as an INVITE to start a call, it acts as a user agent client (UAC). When it receives a request and generates the response, it acts as a user agent server (UAS). Every endpoint does both, switching roles from one transaction to the next. A softphone is a UAC when it registers or dials and a UAS when a call comes in.
SIP messages carry a User-Agent header that names the software and version, for example baresip/4.12. Servers log this value, and administrators use it to spot which client is misbehaving. Some servers also check it to apply device-specific settings, and some administrators hide the server's own version string to give scanners less information.
The distinction between a user agent and a proxy is that a user agent terminates the session and handles media, while a proxy only forwards signaling.
Sources and further reading
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
