In American English, and I imagine in other languages, some numbers have meanings that might not be obvious to a foreigner. Probably the best-known of these is 911, which is what one dials on the telephone in an emergency. When interpreting "call 911" for a non-American Spanish speaker, therefore, it's far more meaningful to say "Llame a la linea de emergencia" then "Llame 911". Telephone conventions in the U.S. also give us "the 411", referring to information on someone or something, and "an 800 number" for a toll-free number (even though there are several other prefixes, such as 888, that indicate a free call). Most number words seem to be nouns, but I do have a friend who will sometimes "*69" (pronounced "star sixty-nine") someone, taking advantage of a feature available on many phones allowing one to dial the number of the last received call.
Outside the realm of telephone services, I can think of one that may not be well-known in states other than California: "420" for marijuana. For a long time I was under the common misconception that this is from the penal code section for marijuana use in the state, but this is not the case; you can read the true origins of the term here.
I was inspired to write on this subject when it occurred to me that my foreign readers might not know the meaning of "Grammar 101" in my previous post. When the name of a subject is followed by the number "101", it refers to a basic lesson in that subject. This comes from a system used by universities in which classes are named by the department followed by a number, with 101 being the introductory course. The higher the number, the more advanced the class. Interestingly, however, none of the colleges and universities I have attended actually use this system; it seems more common, at least these days, to give single-digit numbers to basic courses. Nevertheless, the term has stuck.
And though this isn't a number word per se, I have to add, on the subject of university course numbering, that I find it absolutely charming that the undergraduate quantum mechanics course at the University of California, Berkeley is Physics 137. If you don't know why, look up "fine-structure constant".