Twisted-wire pair (TWP) loops are used as one of the primary communication channels in digital subscriber line (DSL) networks. As part of the development of channel noise models, a transmission-line-based model for predicting the terminal response of a single TWP to an illuminating plane-wave electromagnetic field is presented. Closed-form analytic approximations of the terminal response valid for an arbitrary direction of incidence and polarization of the incoming plane-wave field are provided. A worst-case model with the very same purpose has been previously proposed in the DSL literature; however, a worst-case approach only provides partial information about the field coupling mechanism. The goal of the presented transmission-line model is to introduce a more robust yet efficient approach for characterizing radio noise interference in DSL systems. Computed results for two TWP configurations of interest are presented together with measured results for one of them.