Microsoft sign-in verification phone call

"This is Microsoft sign in system, if you want to sign-in please press the pound key to finish"
What is the pound key on the mobile phone keypad?
It is the key "#".
Why is it called the pound key?
"In the US it's often called the pound key, because it has long been used to mark numbers related to weight, or for similar reasons the number sign, which is one of its internationally agreed names. Elsewhere it is commonly called hash, a term dating from the 1970s that may have been a popular misunderstanding of hatch."