You can refer to the following for serial communication circuit DIY examples,
https://www.beyondlogic.org/
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To interact with the user at the other end of the telephone line,
you can use this MT8870 DTMF IC to decode the dial tone.
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You can also design a ring detector using a couple of diode and transistors,
Available in the website too.
You can design detector for progress call tone to add-on more functions.
Most importantly, you need a microcontroller to control
all your devices and interface between user and computer.
I strongly recommend PIC from microchip (example PIC16F877a).
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It is easy to learn and there are alot of technical support in the internet.
Enjoy your project,
Siong Boon