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
A
Abhayapur 03669
B
Baithalangshu 03677
Barama 03623
Barpeta Road 03666
Bihupuria 03759
Bijni 03668
Bilasipara 03667
Bokajan 03675
Bokakhat 03776
Boko 03621
D
Dhakuakhana 03758
Dhemaji 03753
Dhubri 03662
Dibrugarh 0373
Digboi 03751
Diphu 03671
G
Goalpara 03663
Gohpur 03715
Golaghat 03774
Guwahati 0361
H
Haflong 03673
Hailakandi 03844
Hajo 03664
Hojai 03674
Howraghat 03676
J
Jorhat 0376
K
Karimganj 03843
Kokrajhar 03661
M
Maibong 03670
Majuli 03775
Mangaldoi 03713
Mariani 03771
Moranhat 03754
Morigaon 03678
N
Nagaon 03672
Nalbari 03624
North Lakhimpur 03752
R
Rangapara 03714
S
Sadiya 03756
Sibsagar 03772
Silchar 03842
T
Tarabarihat 03665
Tezpur 03712
Tinsukhia 0374
U
Udalguri 03711
UDARBONDH 03841

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 Assam as a part of our STD|ISD category.

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