When to pronounce # for pound, sharp, hash or hashtag? [duplicate]

How to pronounce # in a proper way?

Currently, I know it's used to pronounce "pound" in US English, "hash" in British English, "sharp" for C#--a programming language, and number sign to list items. Not sure if I'm right or not.

Also, I lives in non-English speaking country and many pronounce "hashtag" for this symbol, though I'm suspicious about this.

I'd be glad if you can list more examples about this.

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Update: To be clear: The question is about the pronunciation, not how it came from.

The name of the symbol in AT&T patent filings is "octothorp," but no one ever says this. If it precedes a number, say "number" as in "#2 pencils." If you're talking about a telephone key pad or if it follows a number say "pound" or "hash" (if you are using US or UK English respectively) as in "enter your password followed by the # sign" or "a 5# bag of sugar." If it's in a tweet say "hashtag."

For speculations on whether # symbol is called "pound" because it replaced the £ symbol in telegraphy and telephony I recommend the following: language log

In Britain it's generally pronounced hash, and in America I believe generally pound. Pound in Britain more commonly refers to the currency. Pronunciation varies depending on the context.

In a tweet it would be pronounced hashtag as is "Off to the bake shop #buyingsomecake" which would be pronounced "Off to the bake shop, hashtag buying some cake"

In musical notation it would be pronounced sharp, as in "Can you play that in the key of F#" - "can you play that in the key of eff sharp"