Are they doing dual writes from the phone? That is, updating datacenter A and B (and possibly C) at the same time? If so, then yeah this will cut down on their need for backhauling between datacenters. If they are only using the phone as the arbiter of truth during a datacenter failover, then no they're not saving any money. You're going to have a circuit or transit in place and that has a monthly fee at 95/5 or flat rate, not per byte transferred.
