500N - that was known as 'phone phreaking'. There was also the famous 'Cap'n Crunch whistle', which came free in cereal packets in the US. Some kid discovered that it was the exact frequency used by single tone dialling systems and it was possible to bypass the toll system by blowing the whistle at the appropriate rate.....
In the UK, before DTMF tone dialling, in the old 'button A and button B' public phone boxes, it was (allegedly!) possible to make free local calls by tapping the cradle to simulate the number of pulses caused by the dial. Long distance calls (allegedly!) required a special code to be dialled, knowledge of which was passed around amongst university students during the early 1970s.
The solution was simple - just reduce the cost of phone calls so that there was no point in phone phreaking!