There are two options
1. SMS based booking
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For SMS Bookings, send a Booking SMS in specified format with information of Date of Journey, Class , Train Number and Passenger Details. You shall receive Seat Availability.
In...
more... the second SMS make the Payment, which can be initiated in either two ways:-
Using Mobile Money Identifier MMID* and one-time password (OTP) as received from Bank for registered Mobile Number.
Using PrePaid Wallet Cards or mWallets.
Booking confirmation SMS is sent by IRCTC.
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2.USSD (Menu-based dialling)
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SMS Services are currently offered by:-
Airtel Money
Dial designated number, you shall receive a Menu Based Screen dialling. Select 'book tickets' option, select 'reservation' (user is prompted to enter his IRCTC user ID once only), enter details (date of journey, station, train no, class, etc)
Enter mPIN for authorizing payment through mobile wallet
Booking confirmation SMS is sent by IRCTC