 The MCU was responsible for looking after all of its Registers and deciding from the dialled information where the call was going to be routed.The local exchange numbering system would be available to the MCU via threadings in the cyclic store so the MCU would be able to read this information and in this way, all exchanges could be configured as required.If a call was identified by the MCU as internal to the exchange, usually by the first digit, then the MCU would tell the Register to come back when the complete number had been dialled.