SIP User Agent is a logical entity at VoIP endpoints capable of initiating and answering calls between peers. SIP user agent consists of User Agent Client (UAC)and User Agent Server (UAS). The SIP User Agent component, when integrated into a device, can enable the device to originate and terminate calls with or without a SIP proxy server. Moreover, this architecture enables addition of features and intelligence to VoIP endpoints without affecting rest of the infrastructure.

SIP User Agent APIs provide interface to services from SIP protocol stack. It provides a level of abstraction between applications and SIP core engine. 3rd party developers can write their own SIP based applications by using SIP UA API calls.