STD (Subscriber Trunk Dialling) is a telephone numbering system that permits subscribers to dial trunk calls without operator's help. The term STD was introduced when it first became possible for making domestic long distance calls. Area codes are used to identify the location of the subscriber telephone connection. Area codes are preceded by a '0'. The '0' is an India's NDD (national direct dialing) code used for making statewide domestic calls. If you are dialing an inbound international call and phone number starts with '0', then you will need to remove that '0 '.
In India, Landline telephone numbers are 6, 7, or 8 digits long. The total length of phone number and correspond area code must be 10. (i.e.,) The total value of subscriber number and STD code of area code numbers should be in 10.