Which we know is untrue.
You yourself stated that the broadband side is straight through, i.e. the DSL and phone signals are present all the way from the MSAN/DSLAM to your modem. A filter does not and cannot make a difference to the broadband signal attenuation.
If a phone is connected to such a circuit without a splitter two things can happen. You may not hear the signals themselves, but it's worth listening to see if you hear anything at the point you switch your modem(-router) on.
More importantly the signal-carrying wires of the phone and any extension cabling easily pick up electromagnetic noise in their environment. Any ring wire present also being a prime factor in noise pickup.
Filters, as you say, filter the high-frequency signals coming down the line. They also prevent any picked up noise on the filter to phone wiring from getting back onto the line to the MSAN/DSLAM.
Good modern filters also make sure noise picked up by ring wires are prevented from getting onto the line. Cheapos may not.
None of the above affect the signal attenuation!. They affect the Signal to Noise Ratio which generally we don't see, and because they affect that they also affect the Signal to Noise Ratio Margin (post-sync), which is what we do see reported by our modem(-router)s.
The filters are in the PHONE side; and are High-Frequency Reject Filters/circuits, to prevent the ADSL/VDSL signals from being attenuated by the PHONE loadis simply complete rubbish and the sort of thing that leads to internet myths.
As for connecting a phone directly to an NTE with no router on the circuit, and not getting any problems, something you stated twice in that post - ?????? What else would anyone expect?
In an earlier post you said "The line is normally very clean on the 17070 QLT". Short of thunderstorms in the area it should be always very clean. If yours isn't, I suggest you have a problem.