What is STD code number?

STD in telephony means Subscriber Truck Dialling. This term was used in Australia in telephony during the transition from operator connected trunk call (out of local exchange area) to full national dialling as we know it today. If a caller was calling by STD then they dialled the called telephone number themselves. When an STD call first connected a series of 7 rapid 1000Hz tones of 1ms duration were sent to the caller and called party to notify them the call was via STD and that there would be no operator intervention to advise them of the duration of the trunk call. If the call was connected my a manual operator, he / she would interrupt the call every three minute and advise the paid period of 3 minutes had expired, and offer another three minute paid connection time. These manual calls were charges in 3 minute increments, while STD calls were changed by the second.