It was mistakenly called “pound sign” because on British keyboards that is where the “L” thingie was.
Why would a £ sign be on a phone?
No, it’s called a pound sign because the symbol has been commonly used for a century to represent dry weight in pounds on container labels, especially agricultural containers.
Back to school with you, grumpy.
I always called it a sharp sign.
It’s the number sign…
You’re all wrong. It’s an octothorp.
Wiki on octothorpe: “Origin disputed. Reportedly a jocular coinage by Bell Labs supervisor Don Macpherson in the early 1960s, from octo- (“eight”), with reference to its eight points, + -thorpe (after 1912 Olympic medalist Jim Thorpe, in whom Macpherson was interested). However, Doug Kerr  attributes octatherp to a practical joke by engineers John C. Schaak, Herbert T. Uthlaut, and Lauren Asplund upon himself and Howard Eby.
The Merriam-Webster New Book of Word Histories (1991) supports octotherp as the original spelling, and telephone engineers as the source.”
“FIG. 1 shows a pushbutton pad 1 … The pad 1 provides keys for numerals 0 to 9, while the sextile or asterisk (*) key is decoded to provide a decimal point and the octothorp (.music-sharp.) key generates a command to send the contents of the memory unto the telephone line through a send circuit 7, a coupling circuit 8 and the hybrid network 2.”
Cool! So, from the date on the patent, if it is on a phone after 1975 (or is it 1973, the date of the filing?), it is an octothorp. You have shown me that context is the key. On a phone it’s an octothorp, on a music staff it’s a sharp, and so forth.
Register with Wiki and update the article and include the patent link as a reference. It’ll make that article more complete.