There was an anti-tapping modification made to the payphones. I don't remember how they worked. In the early eighties most of the payphones in the town I lived in were pulled out because of vandalism.
The red box pay phones were called PCS, Public Call Stations. The phones you would find at hotels, hostel etc were call SCB or Subscriber Call Box. The owner of the premises paid the "rental" for these making local calls free. These phones were the same as a PCS, except for a strap between two terminals on the dial. The strap would make the switch operated by the coin balance in operative, meaning no coins were required for local calls.
When making a toll call the operator would listen for the different sounds made as different coins went through the coin mechanism, a bell, a coil and I can't remember what the 3rd sound was. When the operator was satisfied by the number and value of the coins the call would be put through. Remember that you paid in 3 minute segments. When the time was up you would hear the operator again wanting more money to be inserted.
The other trick was call home from a PCS after work without pushing the "A" button. Until the "A" button was pressed the microphone would not work. If the person receiving the call was aware that the call was from a PCS the order could me made, bread and milk etc. Then you would push the "B" button and get your coins back.
This all ended when the phones in red boxes were phased out.
The current phones use answer reversal for billing. When the call in answered the exchange would change the polarity of the line.