Telephony terminologies

POTS (Plain-Old-Telephone-Service) was created in 1876.

ISDN (Integrated Services Digital Network) was introduced in 1988. ISDN comes in two forms: the basic rate interface (BRI) and the primary rate interface (PRI)

PRI can transfer more data, making it easier to transfer things like HD audio and video and more suitable for enterprises.

PSTN (public switched telephone network) is simply the global aggregate of all these interconnected copper telephone systems.

SIP (Session Initiation Protocol) was introduced in 2000.

Session Initiation Protocol is a set of communication standards that allow (for the most part) the setup and termination of voice or video calls. SIP allows voice traffic to be carried over data networks, including the internet. SIP is considered a type of VoIP.

VoIP (Voice over Internet Protocol) is an overarching term for the technology included in all IP based telephony.

Over-the-Top (OTT) VoIP, services such as WhatsApp, require both calling parties to have an active data connection and carry calls entirely over data networks.

The beauty of SIP is that it can be used to send calls to and from the PSTN, using media gateway.

A SIP Trunk is used to transfer a call between its origin and destination using the Public Switched Telephone Network (PSTN) or in the case of a Voice over Internet Protocol (VoIP) call, the internet. It describes the process of allowing multiple callers access to the same telephone service by sharing a line that can handle multiple calls instead of providing an individual line for each call.