Apple is today celebrating an important milestone: the sale of its one billionth iPhone. Despite iPhone sales declining over the past two quarters, Apple is still selling enough devices to have reached this milestone in the nine years since the original iPhone launched.

At a meeting with employees at Apple's headquarters in Cupertino, CEO Tim Cook stated that "iPhone has become one of the most important, world-changing and successful products in history." And he's not wrong: the first-generation iPhone, which launched in June of 2007, essentially started the smartphone revolution.

Looking at iPhone sales data paints an incredible picture. Despite the nine-year history of the iPhone, Apple has sold just under half a billion iPhones in the last two years, which illustrates the huge growth Apple has achieved.

While sales are declining year on year, Apple is still posting impressive figures for each quarter. In the last three months the company sold 40.4 million iPhones, which is down on the 47.5 million devices in the same quarter last year, but enough to post $24 billion in revenue from just the one product. In comparison, Microsoft's entire quarterly revenue was $20.6 billion.

Apple will see its typical spike in iPhone sales next quarter thanks to the release of the iPhone 7, which is expected to be unveiled in September.