For all you youngsters, basically the way that the old Button A/B phone boxes worked was as follows.

Before you inserted any money the dial was partially disabled with only the 1, 9, and 0 working (any other number came out as 1). This was to enable the free calls to 999 and 0 (later 100) for the operator. However, tapping the number on the cradle (2 taps for 2, 3 for 3 etc) sent the same code down the line. Incidently if you wanted to dial 0, you could tap 10, but it was easier to use the dial of course. If you did tap the number, you got straight through with no payment. Certain numbers (especially on the WOO(lwich) exchange could be dialled free of charge - but of course the GPO kept quiet abot that !

When you inserted your 3 (or later 4) pennies (the great big round copper ones nearly an inch and a half diameter) the dial became fully active. When you got through, you could hear the other person, but they couldn't hear you until you pressed button A and your coins went into the box. Pressing button B cut off the call and returned your coins.

I knew many mums who would tell their kids to ring them at a certain time, but not to press button A unless they were told. This way their mums could give them any instructions without having to pay for the call. The only problem was that mum didn't really know for certain who she was speaking to.