In early generations of telephones the phone apparatus itself had a small desk built in.
This was most common in wall mounted telephones of the end of the 19th and the beginning of the 20th century.
The illustrations show front and side views of such an antique wall phone with a small slanted desk surface fitted with two ridges to keep paper and pencil from falling to the floor.
Many 20th century telephone booths or call boxes had a tiny built-in desk surface for the convenience of customers.
Vandalism and a higher concern for costs has led to the gradual elimination of these minuscule desk surfaces.