After OS X, I worked on a backend team for AT&T. Their entire mobile network at the time - billing, backend, customer service notes... ALL of it was in NeXTStep being streamed from centralized servers out to basic PCs running Citrix.
It was wild to know NeXT had made inroads so many places. I imagine that is why Steve had any sort of relationship with AT&T when he pitched the iPhone and got them to do it. They already saw he could deliver for them on a B&W NeXT-based product used well into the 2000s.