presumably this toroid core is the rein filter
It is a common-mode filter.
An xDSL service exists between two analogue transceiver units, the xTU-C and the xTU-R, connected together by a radio frequency transmission line consisting of a pair of twisted wires carried within one or more sequentially connected cables. The analogue transceivers operate in differential mode. Think of one moment in time (if you like, consider the circuit "to be frozen" in time) and we look at the state of the circuit. The circuit consists of two wires, one twisted pair (with wires labelled A & B), and at that "frozen" moment in time let's say that the the electrons in the wire labelled A are "pushing" from the xTU-C to the xTU-R whilst the electrons in the wire labelled B are "pushing" from the xTU-C to the xTU-R. That is differential mode operation. Now consider the cable carrying the twisted pair of wires linking the xTU-C to the xTU-R. In a real world situation that cable has significant length and can pass through regions where there are (legitimate) high strength RF signals, pulsed electromagnetic-fields, switched high-current inductive loads (to name three possible sources of interference to the xDSL service.) As the xDSL service is carried over a twisted pair, whatever interference that is coupled into the A-wire of the pair is also coupled into the B-wire of the pair. The interference, therefore, appears in common-mode on the pair. Hence the usage of that common-mode filter. It rejects any common-mode signal on the pair whilst allowing the differential-mode signal on the pair to pass unhindered.
