By Grant Ferrett
BBC Africa analyst
Africa is the world's fastest-growing mobile phone market, a new report says.
Mobile phones also work in some rural areas
The International Telecommunication Union says more Africans have begun using phones since 2000 than in the whole of the previous century.
There are now more people using mobile phones across the continent than traditional, fixed lines.
However, only about half of sub-Saharan Africa is covered by a mobile signal, and many people remain too poor to buy their own phone.
Mobile phone companies are one of the great success stories of Africa in recent years.
Use of mobile phones has been increasing at an annual rate of 65%, more than twice the global average.
This remarkable expansion is, in part, a reflection of the wretched state of many fixed line networks throughout the continent.
They have often been confined to cities and have suffered from decades of under-investment.
Customers have embraced the opportunity to have reliable telephone service, largely free of government interference, and at a relatively cheap price.
The fact that mobile phone calls can be funded on a pay-as-you-go basis makes it easier for less well-off customers to budget.
In countries such as Somalia, which has had no central authority for 13 years, the take-up has been particularly swift.
It now has four mobile phone networks and, at about 50 US cents per minute, offers the cheapest international calls in the region.