Yeah, but (e.g. looking at http://www.census.gov/idb/worldpopinfo.html in 2014) 6bn people means that literally everyone over age of 14 has a mobile phone (plus about 100 million of those under 14, too). Even accounting for all the phones first-world children have, it seems like quite a stretch.
It's not that unreasonable. The iphone costs apple only slightly more than $200 to manufacture per unit and there are obviously even cheaper phones out there.
