Jan Koum, co-founder and CEO of Facebook‘s WhatsApp messaging service, says the idea for the company he co-founded with Brian Acton in 2009 came about so he could stop missing calls on his new smartphone.

“It started with me buying an iPhone,” Koum told an audience of several hundred Silicon Valley veterans gathered for an event this week at the Computer History Museum in Mountain View, California. “I got annoyed that I was missing calls when I went to the gym.”

He and Acton then built an app that could let their friends know whether or not they were available, thanks to an easy-to-use feature called “Status.”

“We didn’t set out to build a company. We just wanted to build a product that people used,” Koum said late Wednesday, during an onstage panel discussion that preceded an advance screening of a new documentary called “Silicon Valley: The Untold Story.”

The app didn’t take off right away, even though it was accepted into Apple’s App Store, Koum recounted.

“We were so excited when it launched,” he said. “And so disappointed when no one used it.”

That soon changed, however.

By 2014, WhatsApp, thanks to its easy-to-use interface and uncluttered design, had more than 400 million users globally.

Corporate suitors like Facebook soon came calling.

When panel moderator Michael Malone asked Koum what he remembered most about the day he agreed to sell the company in early 2014, Koum drew a blank.

“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.

Ultimately, Facebook agreed to pay more than $19 billion to acquire WhatsApp, turning both of its founders into billionaires.

Last year Koum sold $2.3 billion in Facebook stock and WhatsApp reached 1.3 billion monthly users.

Malone asked him why he still goes to work, since it’s like Koum “won the lottery.”

“We still have a lot of people who don’t use our product. We want to convince them,” Koum replied. “We still have problems to solve.”

When asked by CNBC after the panel what it was like since Acton left the company last year, Koum answered, “We miss Brian.”