I don’t think I ever use the phrase “number sign”, though I do call it a “pound sign”.
More relevant to the OP, though — If I see an address such as “999 Smith St., Apt #2B”, I would read it as “nine ninety-nine smith street, apartment number two bee”.
Avoirdupois, as was earlier pointed out, not averdupois. Which, oddly enough in describing the English system of weights and measures, comes from the French (goods of weight). Or maybe not too oddly, since we did borrow a word or two from the Normans…
It was always the number sign (if seen alone) to me, or the pound sign if the context indicated so. “Hash mark”, sometimes, but rarely.
Yeah, well Easterners are weird. 
In Germany, a telephone salesman called it the clash (London calling…). At least I’m pretty sure he did. He might have been referring to the asterisk, but I think it was the #. In other places in Europe I heard it called the sharp.
Interesting.
Microsoft invented a computer language called C#
pronounced “SeeSharp”
now I know why.
I always refer to the # sign as pound sign.
But I have heard of hash mark.
In the UK, it’s almost exclusively called “hash”.
Calling that symbol a “sharp” goes back long before MicroSoft. It’s the indicator in written music to increase the listed note a half-tone…instead of playing a C, you’d play a C#, which is (on a piano) the black key immediately to the right of the C.
That’s just a rumor spread by Apple to discredit Microsoft. 
I have always included the # sign when my address was an apartment (for instance, 999 9th st #901). I have no idea where it came from, but was probably taught to me by my parents or my teachers.
Bob
I had many discussions with American business colleagues about this when I was writing technical documents. Problem was, that I was writing for an international audience; they were arguing that American usage should predominate. Calling it the “pound sign” in communications with our British/Canadian/Australian/New Zealand/Indian colleagues meant that we’d run into the problems the OP faced: thinking it was the sign that meant “pounds sterling.” On the other hand, the Americans refused to believe that “number sign,” “crosshatch,” “hash,” “octothorpe,” or “sharp sign” would do. They insisted on calling it a “pound sign.”
In the end, if it referred to weight, I fell back on “lbs.” If it referred to number, I used “#.” If it referred to British currency, I used “the stylized L with a slash through it.” (Sorry, I don’t know how to render the sign on this keyboard.) Everybody was happy–well, sort of, the Americans couldn’t understand why the rest of the world didn’t call “#” the pound sign.
In my 27 years with Pac Bell/SBC/AT&T, # has always been called “pound”. The asterisk is called the a “splat”. (I prefer my own term, though; the “neener”.)
Oh yes, the # sign. Growing up in Southern Ontario, I always called it the number sign. It was only with telephone response systems and the touch tone keypad that we started to hear the usage ‘press the pound sign’. (I suspect ‘pound’ for # crept in, partly because so many telephone-response systems were made in the US, but also because it is a convenient one-syllable expression to balance ‘star’ for *.)
I never saw anyone use the trailing-# to indicate pounds of weight until I hit the SDMB. The Jargon File discusses the number sign in its entry about ASCII:
.
That sounds unlikely; at least I’ve never met any of them, and I know a few.
Go to ZIP Code™ Lookup | USPS and enter the address there, and it will give you the actual address just the way the US Post Office wants it.
That’s your best way to make sure that will be delivered OK.
Not quite. They’re different symbols, although they look similar. The hash sign (or whatever you want to call it) has slanted “verticals” and horizontal cross strokes. The sharp sign has upright verticals and slanting “horizontals”.
To get around the name confusion for this symbol, I vote we call it “train track”.
The “#” in the programming language C# is supposed to be a musical “sharp” sign, but because of the lack of that character on most keyboards, it is usually written with a “hash” sign.
Frequently Asked Questions About Visual C# .NET 2002 (microsoft.com)
Usually written: C#
How it should be written, according to the name: C♯
ETA:
C++ programmer joke:
Q: What do you call the “#” sign?
A: The “rap” sign, as in C# (= Crap! ha ha)
Grew up in the midwest.
