What is the "PSTN switch-off" and why is it happening?

PSTN stands for Public Switched Telephone Network — it's the technical name for the old copper phone network that's carried landline calls in the UK since Victorian times. It's being retired and replaced with phone calls that travel over the internet instead (sometimes called "Digital Voice" or VoIP).

The old copper network is reaching the end of its working life, parts are no longer made, and it's expensive and difficult to maintain. Digital phone services can offer additional features and functionality that are not available on traditional analogue networks. Other countries (such as Estonia and the Netherlands) have already done the same thing.