The problem with africa and mobile apps is, that most cellphones there are not capable of running apps. While it would be nice to "capture" the cellphone payment marketshare, it won't work, until africa upgrades its hardware!
I think we should invent cheap smartphones and then spread them there
Design the app for a feature phone using Java ME with most of the grunt done off the device if necessary. Nokia last year released a Symbian S30 variant for something under $20. People don't need to have the latest quad-core, multi-gig, smartphones running on an LTE network to run simple text based apps
KC