A pager only listened. To reach you, someone called a paging service and typed in a number or dictated a message to an operator. A transmitter broadcast it across the city, and among all the messages in the air your pager picked out the ones with its own address. To answer, you found a telephone.

Numeric pagers showed only digits, so people invented codes. 143 meant “I love you”, one, four and three letters. 911 meant “call me now”. Turn the display upside down and 07734 reads hELLO.

Hospitals were the first big users, in the 1950s, and many still are: paging signals reach deep inside buildings where mobile phones lose their signal, and one message can reach a whole team at once.