Subscriber Trunk Dialing (STD) is a telephone system that allows the subscribers to dial trunk calls without the assistance of an operator. This term was used in UK but now it is widely used in India also. The system required that each area have its own STD code which could be dialed by subscribers. They are known as the area codes.
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In India, to dial out of one's unit area, the relevant city code has to be prefixed to the local number. This prefix starts with a 0. Certain numbers are also prefixed with 95 which are local calls within 500 km. So, the subscriber can easily dial the area code assigned to that place, followed by the actual number and this way one can easily make distant calls.
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