Yeah, but that doesn't explain that much. What kind of coal / mobile phone business? Where did he get financing?

If the business is buying a truckful of coal / handful of phones and going around the village selling retail, he could have gotten finances by himself. But many people do that, and never manage to become billionaires.

Or he had access to some serious capital, usually not available to ordinary people. Either way, there is more to explain, how exactly do you go from the tax office to billionaire status.