What is STD Code?
Subscriber trunk dialling (STD, also known as subscriber toll dialling) is a term for the UK telephone system allowing subscribers to dial trunk calls without operator assistance. The introduction in the UK of subscriber dialling of long distance calls removed the distinction that had existed between Trunk and Toll calls. This term however, is still widely prevalent in India to describe any national call made other than one's local unit. A "subscriber" is someone who subscribes to, i.e. rents, a telephone line and a "trunk call" is one made over a trunk line, i.e. a telephone line connecting two exchanges a long distance apart. Now that all calls may be dialled direct, the term has fallen into disuse.
The term "subscriber trunk dialling" is used in other countries such as the Republic of Ireland and Australia, but the corresponding term in the US and Canada is direct distance dialing.
The system required that each area have its own STD code which could be dialled by subscribers, and although they are now officially called area codes, it is still common to see and hear the old term in everyday use.
In India the term still applies and 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.