Press button B
Many phrases like this one, well known in the past no longer have any meaning. It of course referred to the system of payment in public telephone boxes. The red public phone box was on almost every street corner in the days when there were few telephones installed in private houses and before the invention of mobile phones. In many remote areas of the Uk, they were the lifeline for a village and the only means of communication with the rest of the county. To use a public phone box it was necessary to make a prepayment of the necessary number of coins before dialing the number you wished to call. If someone answered the call then it was necessary to press button "A" to make a connection. If there was no reply then press button "B" to get the coins back. Quite straight forward you would have thought but it did not always work that way. The coins used for many years were pennies. Probably the most common coin in the Uk in terms of usage...