I had the privilege of working on the IT end of one of the first System 12 ITT/Alcatel 1210 switches in the US Virgin Islands (a prototype since local telco was owned by ITT). It may not sound like much now but in 1990 that "look! after this warm restart this existing call is still in progress!" was a big deal, since the smaller PBXs tended to wipe working memory and dump everything on restart. DMS would later dominate the ESS market, but ours was one of the first that could support class 3/4/5 operations simultaneously (trunks, operator positions and subscribers) within the same memory pool which was managed as virtual storage separate from the processors.
Not Erlang but same concept and I'm not sure what the software development environment was, but the thing was loaded to the gills and almost all ops save major software upgrade only required warm restarts which preserved existing connections (perhaps with some delays of digit processing).
Two major problems I remember in those days was a bug in operator position handling where a certain operation created some sort of race condition that brought it all the way down. That was fixed but the other was not so easy... mainly because it was doing triple-duty as class 3/4/5, what functioned as a stack/event queue occupied an insufficient pool of memory to handle extraordinary events. And in that early state of development overflow of the event queue was a fatal cold start error ... from a cold start it took ~15 minutes to come up again.
Murphy's Law supplied TWO great examples to illustrate the problem. One was a severe cable cut, an auger down the street from the telco wrapped itself around ~3,000 pairs and triggered many events. The next was an earthquake... and everyone picked up their phones at once.
Zotonic (an Erlang based framework and CMS) was inspired by the movie and referenced it in their introduction video some years ago: https://youtu.be/r9cmWJvXIj4
Here's the (short) silly cut I made of it last summer: https://www.youtube.com/watch?v=UuSZ37vMIks
Here's another (longer) remix: https://www.youtube.com/watch?v=rRbY3TMUcgQ