Mumbai, Oct. 28 -- In September 2002, when Bharti Airtel said it had added 100,000 mobile subscribers in Mumbai in 42 days, Mukesh Ambani saw a challenge. The story goes that, after seeing Airtel's advertisement, Ambani told his team he wanted 10 million subscribers in the city within a year.

It was a startling ambition in a country where mobile phones were expensive and connections a privilege. Now, Jio Platforms Ltd has debuted on the stock market at Rs.1,358 a share, with a market capitalization of about Rs.12.5 trillion. It is India's second-most valuable listed company, ahead of Airtel and behind Reliance Industries, the empire his father Dhirubhai Ambani built.

The listing caps a quarter-century pursuit: Mukesh entered telecom lat...