On Thursday, 21 November 2019 11:22:46 UTC, Dr S Lartius wrote:
The converter or new phone could include a rechargeable battery, perhaps
of the same size as common non-rechargeables, acting as part of a mini-UPS.

BT used to supply a BBU (Battery back up unit) with their fibre network terminals but (a) I don't know how widely the analogue phone port (FVA) was used (b) this has been removed from the universal service obligation and now they don't.

https://www.ispreview.co.uk/index.ph...broadband.html

Owain