Once upon a time, pay-phones/public telephones used to only cost 5p to call someone. Now you hardly get to say much at all for a minimum of 20p. British Telecom must have spent millions on repairing damaged phone-boxes over the years; If they were really clever, they could have given every home free phones like they do in America, and they could have sewn up the market before all the open-competition of the variable phone companies.
These days, most pay-phones where I live either smell of urine so you wouldn't want to hang around long enough to make a call, or they don't work anyway. Now that most people own a mobile phone, they've lost their main purpose in life, but the red ones will always remain iconic in London life - and when you're in a real emergency and your mobile's run out of battery!