There is no area code 999 in the United States, and no country has 999 as an international calling code. Certain places in Yucatán, Mexico, use the 999 area code, but it should not show up unless both the caller and receiver are in Mexico.
Seeing the 999 area code on caller ID outside of Mexico is likely a glitch or a scam. On phone number forums such as 800notes, many posters report receiving suspicious calls from 999 numbers where the caller attempts to sell a service. Some individuals use call spoofing services to call from 999-999-9999 in order to hide their true identities and phone numbers.