In the United States, there is no area code 999, and no country uses 999 as an international calling code. Certain locations in Yucatán, Mexico, utilise the 999 area code, however it should only appear if both the caller and the recipient are in Mexico.
The appearance of the 999 area code on caller ID outside of Mexico is most likely an error or a fraud. Numerous posters on phone number sites such as 800notes claim receiving questionable calls from 999 numbers in which the caller offers to sell a service. Some persons use call spoofing services to conceal their genuine identities and phone numbers by calling from 999-999-9999.