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.
. Pressing button B to get them back often resulted in two coins getting stuck together and not falling through into the receptacle cup as intended. After a few bangs on the black box, they could come through but otherwise, the frustrated caller would go off. The next person in the box could be lucky and their coins could dislodge the stuck coins and they could make a call by pressing button "a" or their own coins would get stuck as well, adding to the blockage.
Then perhaps along came an enterprising young boy who would manage to dislodge the blockage by various means. Most youngsters "tested" the phone box on every occasion. Often it was just a question of pressing button "b" and retrieving the coins left behind by a caller who had been frustrated in making a call and dashed off without retrieving their money. Otherwise, a few sharp bangs on the box would do the trick, or the judicious use of a penknife, the necessary adjunct to any schoolboy's pocket then, could release stuck coins. Didn't work every time of course, but it was always worth a try, and if you are sauntering along with nothing else to do.......
