I want to make a VoIP ATA (Analog Telephone Adapter) Device using Raspberry Pi, furthermore, I also want to add FXS ports to the Raspberry Pi. Kindly, tell me whether it is possible or not. If Yes, then how?
Here is the Reference Link
This is probably possible, but not with the Pi alone.
You will need to design and build some external circuitry to convert between telephone line audio (which apparently runs at 48 volts) and audio signals which the Pi can produce. Also it looks like the Pi has no audio input, so you might need to either add a USB audio device or use an analog to digital converter that the Pi has to read the audio signal coming in from the phone line, if it can be polled fast enough.
You might have better luck with a board that has a real microphone jack on it already, instead of the Pi.
Then on the software side you need to attach the audio out, whatever you are using to get audio in, and any circuitry you need to open/close the circuit or send special ring voltages to your VoIP software of choice. Working out how to write that driver code is going to depend heavily on what physical circuit you actually build and what VoIP software you want to have talk to it.
That link above has a design for a line-level audio to phone audio conversion circuit which may help you get started. You could also take the circuitry part of the project over to the Electrical Engineering StackExchange site.