User Agent Server-
In VoIP, a server application in an SIP system that accepts the requests from a UAC and generates an accept, reject or redirect response on behalf of the user. The combination of the UAC and the UAS is called the SIP user agent. The SIP user agent allows peer-to-peer calls to be made using a client-server protocol.
Source:http://www.webopedia.com/TERM/U/UAS.html