The Natural Philosopher wrote:

Remember that an on-hook telephone is IIRC a shorted phone.

YDNRC. An on-hook phone looks like a highish impedance, essentially the
impedance of the bell or ringer. For a REN of 1 the impedance is about
4 kohm at ringing frequency. At DC an on-hook phone should appear an an
open-circuit. But, as you imply, all this is irrelevant if the
filtering is working OK.

if taking a phone off stats the broadband, the filter
is not in the right place or not doing its job.

Or, as already noted, the DC line current is 'wetting' an intermittent
connection in the exchange line, reducing attenuation and/or
intermodulation of the HF DSL signal.

--
Andy