https://www.pmctelecom.co.uk/telephones/corded-telephones-analogue/corded-home-office-telephones

Computer Telephony Integration-- The CTI, or Computer Telephony Integration tool, is utilized for linking an existing phone system to a computer, or computer network. Crucial info is immediately recovered from incoming calls, routed to the computers, where the data is processed for useful organisation functions. There is another tool called the Key System Unit, or KSU, which contains helpful functions like call forwarding, extension dialing, and choices for using voice mail. VOIP systems, or Voice Over Internet Protocol, permits users to communicate with other VOIP systems and use digital information through the Internet.