In a phone box, in most semiurbanised parts of the UK before about 1964, to make a phonecall, you went into one of those iconic red phone boxes, and put your pennies into a slot in the black box in front of you. You then picked up the phone, and dialled the number, say SWI3067. After a bit of clicking, you heard the phone ring tone- brrrr...brrrr, brrr...brrr... Then she answered (you hoped). "Hello?" It was her mother (of course). You then had two choices. Press Button A. That connected the call, and you could talk to her mother, who would probably put the phone down and your pennies would have been wasted. Or press button B and get your pennies back, and try again later. I don't know if it worked the same way reciprocally, not being female and trying to call a male (if any ever did).
When I joked about "button A, button B" with my mother years ago (she died in 2000), she didn't understand what I was talking about. She never used a phone box, because she didn't know anyone who had a phone.