This is how founders of WhatsApp got the idea to start the messaging service

WhatsApp Chief Executive Officer and co-founder Jan Koum delivers a keynote speech at the Mobile World Congress in Barcelona February 24, 2014. The world's biggest messaging service WhatsApp, which Facebook has just bought for $19 billion, will add voice calls to its product in the second quarter of this year, its chief executive Koum said on Monday. REUTERS/Albert Gea (SPAIN - Tags: BUSINESS TELECOMS) - GM1EA2O1PBW01

The problem that Jon Koum missed calls when he was at the gym got him the idea to found WhatsApp, the co-founder and CEO of the Facebook-owned company said on Wednesday.

Talking to an audience of several hundred Silicon Valley veterans gathered for an event at the Computer History Museum in Mountain View, California, Koum said that he was annoyed that he was missing lot of calls when he was at the gym. "It started with me buying an iPhone," he said.

He along with Brian Acton in 2009 decided to solve the problem by building an app which would let their friends know if they are available or occupied with something. And, the weapon in WhatsApp’s armour was ‘status’—an easy-to-use feature.

"We didn't set out to build a company. We just wanted to build a product that people used," CNBC quoted Koum.

The app, however, was not an overnight success despite being accepted into Apple’s Play store. "We were so excited when it launched. And, so disappointed when no one used it," he said.

Fortunes changed quickly and by 2014 the app had more than 400 million users thanks to its clutter free and no-nonsense design.

In the same year, Facebook bought the company in a record USD 19 billion deal which made both co-founders overnight billionaires.

When asked what was the most he remembered about the deal, Koum said that even though it’s blurry he could recall sitting in the room with lawyers. "It was all a blur. I don't remember any of that except being in a room with lawyers for three days straight," he said.

When CNBC asked what it was like since Acton left the company last year, Koum answered, "We miss Brian."

Currently, WhatsApp has more than 1.3 billion users across geographies, second only to its owner Facebook which has over 2 billion users in social media space.