In section 3 of the Grounded Activity book (Inventions), on pg. 14 there are some interesting facts about Dr. Alexander Graham Bell, the inventor of the telephone. Here is another one.
The Bell telephone was sometimes referred to as Ma Bell (as in Mother Bell) because the first operators were female. Ma Bell also referred to the female voices of Mary Moore, Jane Barbe and Pat Fleet who were the three women who made the first recordings for the Bell Telephone Company.
This is the kind of telephone Billy and Marie used in the story Grounded. The line had to be shared with up to 8-10 other families.