The TAS is a back to back SIP user agent that maintains a call state. The TAS contains the service logic that provides the basic call-processing services, including digit analysis, routing, call setup, call waiting, call forwarding, conferencing…etc.
The TAS provides the service logic for invoking the media servers to provide the appropriate call progress tones and announcements. If the calls are originating or terminating on the PSTN, the TAS provides the SIP signaling to the MGCF to instruct the media gateways to convert the PSTN TDM voice bit stream to an IP RTP stream and to direct it to a the IP address of the corresponding IP phone.