Billionaire Apple investor Warren Buffett has finally upgraded from a $20 flip phone to an iPhone.

"My flip phone is permanently gone," Buffett told CNBC's Becky Quick during a "Squawk Box" interview on Monday. The Berkshire Hathaway CEO and chairman spoke two days after he released his annual shareholder letter.

"I've been given several of them, including [from] Tim Cook."

Once a fan of the $20 Samsung SCH-U320, Buffett is now using Apple's latest iPhone 11. He's been a longtime user of the flip phone, even though Apple is Berkshire Hathaway's third-largest business, behind insurance and railroads. Now, he said, the flip phone is "permanently gone" even though he admitted he mostly uses the iPhone for phone calls.

"You're looking at an 89-year-old guy who's barely beginning to get with it," Buffett said, though he doesn't use "all its facilities like most people."

"I use it as a phone," he said. Buffett said in the past that he uses an iPad to check stock prices and do research.

Cook said in 2018 he would fly to Omaha, Nebraska, to help Buffett set up his new phone.

Buffett said earlier Monday that Apple is "probably the best business I know in the world." Berkshire owns roughly 5.5% of Apple, according to Buffett. Berkshire owned more than 245 million shares of Apple, worth nearly $72 billion, according to a Dec. 31 filing with the government,

Apple shares are up about 80% over the past 12 months but were down about 4% Monday over worries about how the coronavirus outbreak was affecting its operations in China.