Voice over Internet Protocol (VoIP), also known as IP telephony,

is a set of technologies used primarily for voice communication

sessions over Internet Protocol (IP) networks, such as the

Internet. VoIP enables voice calls to be transmitted as data

packets, facilitating various methods of voice communication.

Voice over IP -

https://en.wikipedia.org/wiki/Voice_over_IP

Plain Old Telephone System (POTS) -

https://en.wikipedia.org/wiki/Plain_old_telephone_service

Retirement of circuit-switched public switched telephone network

(in the UK) -

https://en.wikipedia.org/wiki/Telephony_copper_plant_retirement_in_the_United_Kingdom

Session Initiation Protocol (SIP) -

https://www.ietf.org/rfc/rfc3261.txt

Network Address Translator (NAT) -

https://datatracker.ietf.org/doc/html/rfc2663

Session Traversal Utilities for NAT (STUN) -

https://datatracker.ietf.org/doc/html/rfc5389

Voice over IP Companies -

https://en.wikipedia.org/wiki/List_of_VoIP_companies

Voice over IP Hacking -

https://en.wikipedia.org/wiki/VoIP_vulnerabilities