In Germany analog telephone lines are driven with 60V and 1kOhm in series.
So it looks like a current source of 60mA and the power for electronics inside the phone was generate with a Zener, which consume all the remaining current up to 60mA.
And another phone in parallel would not cause any big voltage change, only current change (1/2 of the whole current).
So you can detect a second phone in parallel with sensing the current consumption.

But with old phones without semiconductors inside, you can still listen if connected over a capacitor and then there is no way to detect it.

Peter