This paper investigates the potential inaccuracies associated with using a modal-domain model featuring a real and frequency-independent transformation matrix in simulating electromagnetic transients on power overhead lines equipped with underbuilt wires. Three transmission line configurations are examined: one without underbuilt wires, one with a single underbuilt wire, and another with two underbuilt wires. Comparisons between the simulated voltages computed using the modal-domain approach and a more accurate phase-domain model as a reference reveal that the modal-domain approach leads to inaccuracies in the simulated voltages. These inaccuracies intensify with an increase in the number of underbuilt wires and the soil resistivity. Differences of up to approximately 24 % in the peak values of the simulated voltages were observed, considering the transmission line geometry analyzed in this study. Based on these findings, it is recommended that for simulating transients in overhead lines with installed underbuilt wires, phase-domain models, such as the Universal Line Model, should be employed.