The simplest circuit to translate from RJ11 to my PC
If you want to "study" the signals, and do some FFT stuff on them you must to acquire them, right?

It's a conversion from analog to digital. So, what you need is a ADC that is able to sample at a frequency at least twice the highest frequency you want to see (Nyquist theorem), and you need a converter that has a dynamic range high enough to not distort the signals (in Volts).

A common used converter used for such DIY project is the audio card. In your case it's not suitable because sampling frequency is too low (44,100 kHz and you need MHz I guess).

So, I think that what you need is a similar to a scope, where you can save the acquired data to your PC.