"Similar concerns in the UK forced telcos such as BT, the former national operator, to delay plans to turn off the public switched telephone network (PSTN) and replace it all with fiber infrastructure"
The 'replace it all' with fiber' bit isn't strictly true. FTTP rollout by BT has been happening steadily, however it has by no means reached everywhere and most areas are still served by copper pairs (I should say metallic, 'coz it might be aluminum pairs as well)!. These metallic pairs will be with us for quite some time to come.
BT/Openreach are de-commissioning the telephone exchanges that make up the UK's 'legacy' PSTN*, The 'voice' service from these exchanges is 'combined' with the xDSL signal onto the 2-wire pair in the network, then 'split' back out into POTS and DSL in the end-user's premises.
When an exchange ('Central Office') is turned off, they obviously need to provide another way of making voice calls. Their solution is to deploy a digital VoIP handset or - for home users at least'' - a modem/router unit (a 'Homehub', for example) that has FXS '2-Wire Analogue line' electronics on-board, allowing old analogue phones - and other pieces of equipment - to plug in and still have a 'voice service'. The Homehub unit is responsible for passing the analogue voice+signalling using VoIP, with the IP transported via the DSL or FTTP 'WAN' port on the homehub.
We can get away with this trick as a much higher percentage of properties in the UK are within reach of DSL-over-2-wire than in the US where people/properties are more spread out.
Things are complicated here as BT/Openreach is one of two operators who have a legally mandated 'Universal Service Obligation', meaning they have to provide voice and internet service when asked (If at all possible). It's further complicated by the fact that most voice/internet service in the UK is the same Openreach-provided wholesale 'physical' product, just re-branded by the individual ISPs.
BT and Openreach are supposed to operate separately, with 'BT' being the retailer/reseller, and Openreach being the wholesaler and owner of all the plant/equipment infrastructure - including the increasingly unreliable but still very valuable 'copper network'.
Lastly, as the Exchanges/Central Offices disappear, so does your 'Battery Backup'. Exchange-provided analogue phones on a 2-wire line are powered directly from the exchange, meaning they would work even with a power outage at your premises. With VoIP based service, YOU are responsible for the battery backup, including the FTTP termination box (It's basically an optical-electrical converter).
*That is what I used to do for a living, that and the ISDN.