Telecoms is really no different to computing and the long term shift was to miniaturisation and now more recently to virtualisation. Voice ‘switching’ once a very physical thing requiring actual physical hardware can all be done in software often hosted elsewhere or virtualised itself. So the physical space and footprint required shrink massively.
At the same time the move to converged IP everything and smart personal devices, the general model has shifted from centralised to distributed or “peer-to-peer”. Voice and video is packetised and streamed and can be transported, stored, duplicated and manipulated like any other stream of data.
Finally the shift away from (lots of) copper to (a tiny relatively, fraction of aggregated) fibre in the last mile really means that a plethora of small local exchanges aren’t required. One exchange site can now serve a physical area covered by a dozen smaller exchanges.