Credit-card holders in India became subject as of Jan. 1 to entering a passcode whenever they make a transaction by phone, under a new regulation.

Banks in India must decline any transactions initiated by phone, including those using automated Interactive Voice Response systems, if the customer does not have and use a one-time passcode, the Reserve Bank of India, the country's central bank, announced last month.

Cardholders must register for the service with their banks, which will send requested passcodes only to the customer's registered mobile phone number and e-mail address.