India's Telecommunication Network is the second largest in the world. Taking about wired land line users, it has been a part of life of many Indians from couple of decades. There were around 2.1 crore subscribers of fixed wired landlines until January 2020.

STD (Subscriber Trunk Dialing) and ISD (International Subscriber Dialing) codes are placed before any telecom number either it be fixed landline or mobile sevice. STD is used when you want to call across the cities of India while ISD is used when you want to connect to any Indian city from outside the boundaries of the nation(overseas).

City STD Codes
GANGTOK 03592
GAUZING 03595

And to talk about mobile wireless telecom services there is no city-wise distribution of STD and ISD codes, there is only one national code for each category. STD code for mobile telecom is '0' and ISD code is '091' or '+91' for India. So here scroll and check out the STD and ISD codes of all the cities of state of Sikkim as a part of our STD|ISD category.

For international calls one can use (+91) + STD Code(listed below) + fixed line contact number