Analysis
NAICS 51781 is the 2022 NAICS industry for all other telecommunications, part of 5178 (All Other Telecommunications).
It has only one more detailed code, 517810, which covers exactly the same activities; the definition, index entries and statistics are on that page.
County Business Patterns publishes no national count for it, either because the code is out of the survey's scope or because the figures are suppressed to protect individual businesses.
Official definition
See industry description for 517810.
Source: US Census Bureau, NAICS 2022 Manual.