What would be the challenges with this approach?
1) First build a robust standalone system, that skips the card networks, the banks and the mobile operators. User adoption is not a priority at this stage, most likely you would have trust issues.
2) Expand your base framework to make it an ecosystem by actually building API for merchants and businesses. Users still not the top priority at this stage.
3) Slowly but surely pursue integrations with the semi-online players like Ecocash and all those other guys. It will take a while, but once done, users will probably come in.