In the United States, there is no area code 999, and 999 is not an international calling code for any country. The 999 area code is used in some parts of Yucatán, Mexico. However, it shouldn’t show up unless both the caller and the person being called are in Mexico.
If you are outside of Mexico and see the 999 area code on your caller ID, it is probably a mistake or a scam. On 800notes and other forums about phone numbers, many people say they get strange calls from 999 numbers where the caller tries to sell them something. Some people use call spoofing services to call from 999-999-9999 to hide who they are and what phone number they really have.