Calls from 000-000-0000 are mostly from telemarketers or collection agencies. If the calls are persistant like numerous calls per day, it's most likely a collection agency.
I'm not sure what the laws are regarding telemarketers, but I know that "caller ID spoofing" is highly illegal for a collection agency to use. If you answer and it is indeed a collection agency, quickly report them to your state Attorney General's office, as well as the Federal Trade Commission (
www.ftc.gov).
Both these government agencies are responsible for consumer protection and should always be notified if you are being harrassed by ANY collection agencies (especially ones that resort to illegal means of contacting / harrassing you against the FDCPA guidelines).