One should check via
www.whois.us (authoritative for .us).
Reserved Name Rules:
All domains of any length beginning with 18XX or 1-8XX or 8XX
where X is any number from 0 to 9 inclusive.
All domains beginning with NXX or 1NXX or 1-NXX followed by exactly seven numbers not including hyphens,where N is any number from 2 to 9 inclusive and X is any number from 0 to 9 inclusive.
All domains beginning with XXXXX where X is any number from 0 to 9 inclusive.
Example of reserved domains:
1800abc.us (rule #1)
812abc-def.us (rule #1)
1-805-999-9999.us (rule #1)
212-999-9999.us (rule #2)
19009998888.us (rule #2)
1-212---1-2-3-4-5-6-7--.us (rule #2)
20176.us (rule #3)
90210-Melrose.us (rule #3)
Since SSN domains would all begin with 5 digits in a row, they would violate rule #3, and are thus can't be registered.