Nec code for cable depth

the electrical code for coaxial cable appears to state that direct bury needs to be 24 inches deep and buried in conduit 18 inches. I see on here a response that Comcast depth varies 6 to 18 inches of cover. Can you confirm the  amount of cover is supposed to be 18 or 24 inches for my area or can you point me to the code that allows shallower depth for communication service lines. it appears the utility locate ticket indicate a depth to "point 3" which I interpret as .3 feet or 3.6 inches which is very shallow.