Increasing deployment of digital subscriber line (DSL) technology operating in the long wave, medium wave and sometimes short wave frequency ranges, has raised the question of how such systems interact with rival radio frequency sources and equipment. Two of the major standards bodies in the field, ITU and ETSI, imposed by the standardization of such promising technologies like ADSL, SHDSL and VDSL, have begun to elaborate performance tests for radio frequency interference (RFI) scenarios for inclusion into their standards documents. Existing out-door loops of the copper telephone network can have large dimensions, are often unshielded and sometimes not even twisted, thus, they can act like antennas for both RFI egress and ingress. The RFI egress issue requires basic knowledge of the relationship between a given current distribution I(l) on a twisted pair communication cable and the electromagnetic fleld E, H generated by that current distribution at a given distance d. This problem is reciprocal to the RFI ingress issue, which requires knowledge of the relationship between a disturbing high-frequency electric field E in the surroundings of a twisted pair cable, and the terminal voltage of that cable across a load impedance. This contribution provides the theoretical background to the field theoretical problem and supplies simple yet accurate approximation formulae for the prediction of RFI egress and RFI ingress.