the old landline network did have notable bugs - sometimes they caused service outages - we never noticed because the network as a whole was designed with enoug

the old landline network did have notable bugs - sometimes they caused service outages - we never noticed because the network as a whole was designed with enough redundancy and to not fail silently - you could loose many components of the network, and still have it remain functional, every switch for example deployed ran duplex (there were two computers running in lockstep), you could loose chunks of switching fabric within a switch and still have it be more or less functional, though in a degraded state. Almost every end office had more than one route out, to account for equipment or trunk failures. You are correct, bug free software is possible - but often designing stuff to fail safe, is cheaper than catching every bug, and a better use of time. There is a reason we include hardware watchdogs on stuff still, because the watchdog is cheaper than building bug free software, and still provides the same quality of service.