Complex sentence: Telephone extension cords are typically made with small-gauge multi-strand copper conductors which do not maintain a noise-reducing pair twist, which are more susceptible to electromagnetic interference and have more attenuation than single-strand copper wires typically wired to telephone jacks.
Split into simpler sentences: Telephone extension cords are typically made with small-gauge multi-strand copper conductors which do not maintain a noise-reducing pair twist. Such cable is more susceptible to electromagnetic interference and has more attenuation than solid twisted-pair copper wires typically wired to telephone jacks.