"iPhone has become one of the most important, world-changing and successful products in history. It's become more than a constant companion. iPhone is truly an essential part of our daily life and enables much of what we do throughout the day," Cook said in a statement.
Cook said that the billionth iPhone was sold last week.
"We never set out to make the most, but we've always set out to make the best products that make a difference. Thank you to everyone at Apple for helping change the world every day," Cook said.
Apple declined to comment on where the phone was sold.
— CNBC's Josh Lipton contributed to this report.