Technically, IP Telephony is something associated with packet-switching, while PSTN (Public Switched Telephone Network) is related to circuit-switching. In simple language, with PSTN, there is a direct connection between two communicating voice parties via a dedicated circuit. And, that circuit is completely reserved for a single contact. And, with IP telephony, multiple packets are passing through a single communication line. And, the voice packet of one or more conversations may or may not travel via that same path, as other packets are transmitting data, video, etc. With this, we can determine that the PSTN is something traditional that needs a huge number of assets, and IP telephony is a modern system that makes everything simpler just by the internet.