Not entirely sure, but what you are talking about sounds like a pbx system. These use hardware/software for internal routing answering the phone, forwarding, storing messages and so forth. It generally isn’t inexpensive to implement. There might be something along these lines for systems with between 2-4 lines. If so, stores such as Staples and other office product suppliers would have them.

Most or all residential phone lines are wired to the interface box with 4 wires and almost all residential wiring has a spare set of wires. Cat 3 cable, which is used for most phone line, uses 4 wires. It takes 2 wires to make a phone circuit, so you probably already have one big component needed to install a 2nd line.

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...Tracy

'00 ML320 "Casper"
'92 400E "Stella"