Here you can find a complete list of STD Codes of all various cities accessible in Tamilnadu. The expansion of STD is Standing Trunk Dialing. When dialing anywhere outside the district or City you will have to prefix the STD code before the phone number - for example (044-12345678 for dialing in Chennia). On the other hand when making International calls that is from outside India you need to follow ISD Code (International Subscriber Dialing). The ISD code of India is 091 and that has to be prefixed before the district code and the phone number for example (0091-44-123456789)