Sorry i have to disagree if you remember back to my BT80b install I put the one leg into A and the other leg into Earth phone worked but no BroadBand sync.
I remember that event.
If you think about the circuit which resulted, you had a series connected inductor (with a minimal resistance) in one leg. That minimal resistance was well within the operating parameters for telephony and the inductance had no significant effect in the voice frequency pass band (300 Hz to 3.4 kHz). Hence the telephone worked. However the inductance was sufficient to perturb the VDSL2 circuit such that the transceivers (one in the modem, one in the cabinet DSLAM) were unable to achieve synchronism. The DC continuity of the circuit was maintained, the AC continuity of the circuit was acceptable for telephony but the AC continuity at VDSL2 frequencies was non-existent.
Now consider the hypothetical case where one leg of the pair becomes detached at the pole top DP (where the drop cable is connected to distribution cable). There is no DC continuity whatsoever, hence the telephony service is interrupted. However the VDSL2 service will "just about" operate in a highly degraded state. Why? A VDSL2 circuit (or any xDSL circuit, in the general case) consists of a pair of radio frequency transceivers normally linked together by a "balanced feeder". Call it a RF transmission line, if you like. When one conductor of that transmission line is disconnected, the RF signal will pass through free-space -- just like any radio transmission -- and induce a signal in an "aerial". In a way, you have a pair of transceivers operating via a pair of aerials (the two ends of the disconnected wire) in a very inefficient fashion. So the broadband service can operate in a severely degraded state.
Which is my verbose way of saying that
Eric is correct . . .