Sniffing ADSL data exchange
In order to show this image but just a comment to last question . Yes it could be an ADSL modem, or not. You would then use the decoder to obtain the binary data and attempt to find the bit sync, byte sync and frame sync detector pulses and disable the Tx.
Consider this clandestine patent for sniffing unknown HSTU-R's.

I trust this answers all your original questions.
Note you need to examine the received analog signal level indicator to discriminate direction and verify modulation type only during negotiation period of dial-up otherwise would have to force the demodulator into a known state for ADSL over POTS vs ISDN  vs AoP/AoI/AoL/SSDSL/HDSL/shared RNIS.
Obviously, you need design experience to  communicate this state onto dummy HSTU-R receiver. Ideal is to use same model but that may not be most flexible model to work with. You also need good experience with Logic Analyzer and superior design skills in order to reverse-engineer this communication. Not for the faint of heart.  I can say this with experience.
A complete design solution is beyond the scope of this forum.