Call Set - up Audio Transmission Call Termination Hang Up Tone Hang Up Signal Off - Hook Trunk Line Hang Up Control Off - Hook

An Internet Telephony Gateway Server is a server that integrates the Internet into the Public Service Telephone Network (PSTN). It makes use of the Internet as a new form of media to transmit speech audio between two conventional telephones. This paper examines the software design of the server and identifies the necessary Application Programming Interfaces (APIs) for the server software. Two main groups of APIs are required. The first group of APIs interfaces with the hardware interface to answer calls, decode DTMF tones, check trunk line availability, make and terminate calls. The second group of APIs interfaces with the Internet to provide functionality for establishing connection, packet loss replacement, packet ordering, packet time stamping, compression and decompression.