The growth of mobile connections

"The first commercial citywide cellular network was launched in Japan by NTT in 1979
The milestone of 1 billion mobile phone connections was reached in      2002
The 2 billion mobile phone connections milestone was reached in 2005
The 3 billion mobile phone connections milestone was reached in 2007
The 4 billion mobile phone connections milestone was reached in February 2009"

Source: Original sources not given, but this is from Ajit Jaokar's blog, 17th April 2013

Note - The GSMA say that there were 6.8 billion mobile connections by the end of Q4 2012