"The percentage of Africans who could access a mobile phone leapt from only 10% in 1999 to more than 60% by 2008—far outstripping improvements in other infrastructure (roads, clean water, or indeed landline telephones)" from Jenny Aker writing on the Center for Global Development's Blog
As described in the link cell phones can do a lot of great things. People can get updates on the latest prices, order goods faster, and communicate with their safety net in times of trouble. Cell phones are not a substitute for everything. To work well farmers have to have goods to sell or money to buy things. To complement cell phones Aker suggests there also needs to be improvements in education and health infrastructure. Related Aker is also working in Niger to see if phones can help improve literacy. Early evaluations by Aker and coauthors suggest that phones can in fact help people learn to write and improve their math skills.
I also recall using cell phones to deliver health information. So as Aker points out they may not be silver bullets, but her work suggests a lot of promise for phones.