From the horses mouth -
2018 – Plans to retire the old telephone network within seven years are announced. This will see everyone in the UK upgraded to a digital phone line, laying the foundations for the future
2019 – Salisbury becomes the first Openreach Full Fibre city in the UK. The Wiltshire city has the fastest citywide network in the UK
2020-2023 – Openreach stops selling PSTN based copper products initially at an exchange level and then nationally. Openreach will adopt a phased approach to stop selling copper broadband and telephony products. From September 5th 2023, this will apply across the UK
2027 – Retiring the old analogue phone network. By this time, everyone in the UK will be upgraded to a digital phone line – which will use the broadband network to make calls in a similar way to other Voice Over Internet Protocol (VOIP) technologies like Skype, Facetime, and Microsoft Teams; enabling HD calls, improved functionality and allowing the UK to keep pace with the rest of the world, all while continuing to provide the reliability and reassurance of the traditional landline
https://www.openreach.com/news-and-opinion/2023/openreach-change-telephone-network