ISO/IEC TR 22767:2005 describes how CSTA can be used to provide a subset of CSTA call control functionality, called first party call control, for Session Initiation Protocol (SIP) user agents. The term uaCSTA (for user agent CSTA) refers to transporting ECMA-323 (CSTA XML) messages over a SIP session.

SIP is a control (signalling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.

CSTA standardizes a very powerful and flexible set of application services to observe and control voice and non-voice media calls as well as control and observe non-call related features.

uaCSTA leverages SIP mechanisms to provide a highly featured, robust, and extensible set of features to support applications in the Enterprise environment.

uaCSTA can be implemented by several different types of SIP user agents:

• directly by a SIP user agent on a SIP phone,
• uaCSTA can also be implemented by a SIP B2BUA to augment 3PCC functionality, and
• by a proxy server that is front-ending a PBX.