Telephony is how the audio gets from one party in a call to the other. This involves picking up the audio signal with a microphone and somehow converinng that signal so that it can be carried to the other side and reproduced. Usually this involves a codec that converts the analog signal to digital. Telecomm has to do with the circuits and systems that carry phone calls. TDM - Time Division Multiplexing - the traditional way of slicing bandwidth so many calls can share a high speed line. VoIP - Voice over Internet Protocol - Carrying voice in IP packets across the internet. More efficient use of bandwidth but incurs possible delay and quality issues.