The AT&T Trimline and similar phones from other manufacturers have gone through major changes over the years.
The early ones had cords that plugged in, but the plugs were not the miniature "modular" design used in modern phones.
Early phones had touchtone pads or rotary dials which were illuminated by incandescent bulbs connected to a transformer plugged into an electrical outlet. Power for the light was carried by the black and while wires in the line cord.
Later phones were illuminated by LEDs powered by the phone line and did not use transformers.