Is the ADSL feed in the Exchange left physically connected; or is it disconnected; or otherwise "switched OFF"?
The process of minimum intervention is followed. So the physical jumpering, connecting the exchange located MSAN to the circuit, is left in situ upon an end-user's upgrade from a G.992.5 to a G.993.2 based service. Obviously if that MSAN port is now required for a different end-user's service, the jumpers will be modified appropriately.
Hence it is quite possible for the G.992.5 signal to remain upon the circuit, event though a G.993.2 signal is now being injected at the cabinet. It really depends upon the "loosing" provider's mode of operation . . . if a change of service provider has also occurred.
The cabinet based low-pass filter will take care of the above scenario perfectly adequately.
The low-pass filter actually performs a number of functions:
(1) It prevents the E-side cabling acting as a bridge tap on the G.993.2, D-side based, circuit.
(2) It enables the voice-band signal (300 Hz to 3400 Hz, for telephony) to be extracted from/inserted into the G.993.2 circuit.
(3) It prevents any exchange based G.992.5 circuit, if still extant, from being extended to the end-users CPE.
100% Linux and, previously, Unix.