AskKaka lists the STD code of Lucknow that is 0522. Lucknow is a part of Uttar Pradesh and the ISO code of Uttar Pradesh state is UP.
Lucknow STD code aka subscriber trunk dialing code is 0522 which is mentioned above on this page of AskKaka. We are great resource of finding Lucknow std code of Uttar Pradesh (UP).
Useful Info :-
Please take a note that if you are in India and dialing a landline number of Lucknow then you will have to dial STD code 0522 as a prefix followed by the phone number of a landline user and if you are calling Lucknow's phone number from a foreign country then you need to add +91 (the country code) followed by 522, e.g. +91522 (no need to add "0" as prefix) followed by the landline phone number.
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