In this paper, an efficient procedure is proposed to predict the response of unshielded twisted pairs. The procedure is based on the characterization of a short cable length by a suitable three-dimensional full-wave surface integral formulation. The proposed formulation allows to account for skin effect, proximity effect and radiation losses. The computed impedance matrix representation is used to extract the Π-type equivalent circuit suitable to be implemented as macro in SPICE circuit simulator. Two different approaches to predict the response of long line are proposed and compared. In the first approach, the long line is modeled by simply cascading several SPICE macros, while in the second the long line response is obtained by simple manipulations of the full wave results.