You forget that large carriers have legacy infrastructure and are facilities based…that is: investment in plant is amortized as a cost to maintain the network. In this instance backhaul come into play because that cellular call you make is not end-to-end wireless. Most of that call is backhaul end through that legacy infrastructure that AT&T has had in place well before cellphones were commercially viable. The emergent network design is IMS. END TO END IP. The the all IP Multimedium Subsystem is mode agnostic.