An electronic program controlled switching system is disclosed having a ringing control circuit for testing called lines for foreign potential and excessive leakage before ringing the called phone. The circuit includes bridge rectifiers and transistor circuitry for checking for foreign potentials and ferrod sensors for detecting undesired leakage. An opto-isolator and relay arrangement controls the application of ringing to the called line. Active filter and latch operational amplifiers sense the application of the ringing and the answering of the rung phone. Magnetic latching relays establish successive test and ringing configurations of the ringing control circuit components under control of a signal distributor and a program controlled processor. Ferrod sensors convey ringing control circuit information to the processor via a scanner.