Because the phone is connected directly to the incoming telephone line,, it clearly can affect the DSLsignal if it's defective or even badly designed. The ultimate defect would be a complete short circuit of course, which would stop everything working. But varying loads, resonances, etc. could affect the DSL in various ways.
My own understanding is as follows but as always, my 'electronics' knowledge is somewhat stale, so happy to be educated/corrected...
I thnk that the modem is usually connected directly to the incoming line, but the phone is isolated with low-pass filters on both conductors, ie inductors/chokes, which thus prevent the phone from 'seeing' the DSL signal. And if it cannot see it, it cannot attenuate it.
Even in the case of a direct short, a high frequency (DSL) signal could still be present on the 'line' side of a low-pass filter, though I doubt that is happening here as the phone would not be working.
It is true that if the inductors were defective, the phone would be able to affect the signal. But the most likely failure mode of such a component would be open circuit, in which case again, the phone would not work at all.
That said, in any such environment all conductors and surfaces are acting as antenas and reflectors that will modify the effects of whatever interference exists, so I would expect some difference whenever a phone is connected - maybe better, maybe worse - but not too significant. I assumed we are talking here about a more significant effect.
Or as said, I may just be wrong.
