Finnish mobile phone maker Nokia says it has shipped the 1.5 billionth phone running its S40 operating system.

The first S40 phone was sold in 1999, and the software underpinned Nokia's commercial success for years.

"It's difficult to think of a product line with such an impact on the world's population," said Mary McDowell, who runs Nokia's mobile phone division.

S40 is used in feature phones, which can run applications, but lack the computing power of smartphones.

Nokia estimates that there are currently 675 million active owners of S40 phones in the world, and says that it is currently selling about 12 phones of the S40 family every second.

Today's version of S40 is far removed from the original, with some feature phones having functionalities and running applications that until a few years ago were the exclusive reserve of smartphones.

Luring the next billion

And Nokia believes that the S40 line has some way to go.

The company says that the software's efficient management of phone memory and battery power makes it an ideal candidate to "connect the next billion" - people who either don't have access to telecommunications right now, or are too poor to afford smartphones.