Complex sentence: The model 102 telephone consisted of the desktop set (handset mounting) and a physically separate subscriber set ("subset"), a box that was typically mounted on a wall near the phone or on the side of a desk.
Split into simpler sentences: The model 102 telephone consisted of the handset mounting typically placed on the desk top and a physically separate desk set box or subscriber set. This box was typically mounted on a wall near the phone or on the side of a desk.