A three-dimensional full-wave surface integral formulation is used to predict the terminal response of unshielded twisted pairs. The proposed approach allows taking into account the skin effect, proximity effect and radiation losses. The proposed full-wave simulation is validated by comparing the results with those obtained by a software tool based on the finite integration technique, discussing advantages and limitations. The surface integral formulation results obtained on a short length of the line are used to predict the behavior of a longer line. In addition, they are used to extract equivalent circuits suitable for the time domain analysis by any commercial circuit simulator