There is no area code +1 999 in North America (USA, Canada, etc.), nor is there an international country code +999. They are both invalid numbers.
Unfortunately, it is very easy for telemarketers and scammers to send falsified Caller ID data, and it's not even illegal. There are also reports that in some circumstances (including some calls from U.S. military bases overseas) where the actual number is unavailable, the telephone system may insert a bogus number.
As of June 1, 2012 currently no cellphone provider or phone company offers blocking the entirety of 999 or other fake phone numbers that operate in such a manner for collection agencies seeking protection against illegal or malicious intent practices, scam artists or even offenders known to make threatening or malicious verbal assault via harassing phone calls. There are no laws currently limiting the use of such invalid numbers.
One measure that deems limit to action is the constant hang-up but if malicious calls do occur then it's an action of the individual to document, record and report to the police that then will clarify actions on your behalf for the phone company to end all phone calls from a 999, and/or other fake area code numbers!
Note: if the numbers are malicious in intent via threat, profanity according to FCC the Federal Communications Commission is a Federal Offense - depending the number of calls and the severity of the incidents may have a penalty as high as $500,000 or a maximum of 50 years prison sentence in the U.S. Jurisdiction.
There is no area code +1 998 in North America (USA, Canada, etc.). Mexican area code +52 998 (or +52 1 998 for mobile phones) is Cancún. Country code +998 is Uzbekistan.
No such area code. Telephone area codes in North America (USA, Canada, etc.) cannot have '9' as the middle digit.
No such area code. Area codes in North America (USA, Canada, etc.) never have '9' as the middle digit.
There is no North American area code +1 998. International country code +998 is Uzbekistan in central Asia.
The country code and area code of Gulistan, Uzbekistan is 998, (8)67.
The country code and area code of Jizak, Uzbekistan is 998, (8)72.
The country code and area code of Fergana, Uzbekistan is 998, (8)73.
The country code and area code of Khorezm, Uzbekistan is 998, (8)62.
The country code and area code of Samarkand, Uzbekistan is 998, (8)66.
The country code and area code of Kohistan, Pakistan is 92, (0)998.
The country code and area code of Andijan, Uzbekistan is 998, (8)74.
The country code and area code of Bukhara, Uzbekistan is 998, (8)65.
The country code and area code of Karakalpakstan, Uzbekistan is 998, (8)61.
The country code and area code of Karshi, Uzbekistan is 998, (8)75.
The country code and area code of Navoi, Uzbekistan is 998, (8)79.