If it’s in a prerecorded phone machine, then #, also known in Spanish as “la cucaracha” (the roach) or “el cuadradito” (the little square). Of course the pre-recorded messages never call it the roach.
I first encountered this in 2003 in the USA and had to ask a coworker what the heck was the machine talking about. That symbol isn’t used for anything in Spain, outside of telephone keypads and music sheets. I’d seen it before in the USA (1994-'8) but we called it “the number sign”.
And in any other context it’s either the squiggly crossed L used for UK currency, or a capital L used for prices in places in Spain which still use pounds for some items (the actual weight varies between 400g and 500g, not just by region but by - kind of item!)
‘Pound’ is a very US way of referring to the # key. Here in Canada, we always call # ‘number sign’, except in the context of phone menus. Many pre-recorded phone menus say ‘hit the pound key’. I think this is because either they are operating from the States or they were made in the States. I also suspect ‘pound’ provides a nice one-syllable counterpart to ‘star’ for *.
I have never, ever, seen anyone in Canada use # to indicate pounds weight.
Good lord, I’m starting to think I’m the crazy one. But there’s at least one other crazy person - whomever wrote this part of the wiki entry on “number sign”:
The hash key. I always hear my bank’s female electronic voice saying that from when I do phone banking. “Now enter your pin number, and press the hash key.”
My first awareness that it had another name came through stuff like MAD magazine and the like, I think.
“Number sign” is how I learned it, back before I could even type. On typewriters, it was included on the numerals line for the purpose of designating “numbers” as in “Incident #42783”. I didn’t hear it called “pound sign” until the advent of automated phone answering systems … back in the 1980s?