Integrated Services for Digital Network, abbreviated to ISDN, is a set of worldwide communications standards for the digital transmission of voice, video and data services over the PSTN (public switched telephone network). ISDN is set to be switched off in 2025 by BT.
A PBX (private branch exchange) is a telephone system within an enterprise that switches calls between employees on local lines while allowing all users to share a certain number of external phone lines.
Public Switched Telephone Network abbreviated to PSTN, is the collection of worldwide circuit-switched telephone networks that carry analog voice and data over copper wire. This network will be shut down completely by BT by 2025.
Session Initiated Protocol (SIP) trunking is a replacement for traditional ISDN. It facilitates the connection of a PBX telephone system to the Internet. This means that the internet replaces the conventional telephone trunk/line, allowing businesses to communicate using the benefits of internet telephony.
Service Level Agreement, abbreviated to SLA, is a document in which the service provider promises to solve any problems with the service in a certain time-window.
VoIP (Voice over Internet Protocol) refers to the delivery of voice, video, and data communications over the internet, rather than over the legacy ISDN.