JUMP onto the back of a motorcycle taxi almost anywhere between Nigeria and Kenya and the odds are that your driver will be wearing a high-visibility vest carrying the logo of a mobile telephone company. The phones themselves are even more ubiquitous. By the end of this year there will probably be close to 1bn active subscriptions for mobile phones across the continent, almost one for each of 1.2bn people living in Africa. That is a stupendous jump from fewer than 100m in 2005. Yet a slightly closer look shows that although a very large number of Africans have several active phone numbers (they switch SIM cards to get better deals if travelling, calling abroad or those on other networks) more than 50% of people have none at all. Why is it that despite such rapid growth in the mobile phone industry elsewhere in the world, more than half of Africans still can’t phone home?

The question is important because phones and, increasingly, mobile internet access are closely tied to economic growth. Across the continent phones are helping to increase the incomes of groups such as poor farmers by allowing them to check the latest prices for their crops before they sell them to middlemen. The growth of mobile money in places such as Kenya allows migrant workers to send money home safely to pay for school fees that will allow their children to get better jobs. A recent study found that just having access to M-Pesa, the ubiquitous form of mobile money in Kenya, lifted some 2% of the population out of poverty. If half of Africa’s people don’t have even simple phones, never mind smart ones that can get them onto the internet, there is a real risk that they will fall even further behind.

Geography goes a long way in explaining why such few Africans have phones. For a start they live on huge continent (far bigger in reality than it looks on standard maps that use the Mercator projection). If drawn to a more realistic scale the landmasses of China, India and the continental United States would all fit comfortably on the African continent. Beyond its size is Africa’s low rate of urbanisation: some two-thirds of people on the continent live on farms or in small villages. Getting a phone signal across that vast expanse entails not just the construction of tens of thousands of masts, but in many cases all of the services needed by them. These include thirsty diesel generators, since there are not reliable supplies of grid-electricity on most parts of the continent, as well as ways of transmitting calls back from each tower to the rest of the phone network over thousands of kilometres of savannah and jungle. The third factor is poverty. About 40% of Africans live on less than $1.90 a day. Phone companies would not only have to spend a fortune to reach Africa’s scattered poor, they would also not get paid very much for doing so.

Yet there is hope that new technologies will significantly lower the costs of providing phone and data services. Some companies are looking at shrinking mobile-phone masts so that instead of beaming out high-powered signals over hundreds of square kilometres of empty space, they instead provide low-powered signals just where they are needed. This in turn means they can be powered by solar cells and batteries instead of expensive generators. If they are right, Africa’s disconnected half may soon be getting a call.