Simple definition
A SIP user agent is software or a device that initiates and receives SIP signaling.
Purpose and use
A SIP user agent can send or receive session messages, for example in an IP phone or softphone.
How does it work?
It acts as a user agent client when initiating a request and a user agent server when receiving one; these roles can change during one call.
Example and verification
During an incident, identify which endpoint generated a response or stopped sending expected messages instead of always attributing failure to the PBX.
Points to watch
Claimed SIP compatibility does not guarantee matching codecs, secure transports or advanced features.
In short
The user agent participates in signaling and negotiation; its configuration must be compatible with the other participants.