Panasonic add on handset for DECT phone system. Model KX-TGB610E with base and wall wart mains adapter.
This can talk directly to a BT phone line (well, via a digital adapter or a broadband router's phone socket) if you have the right wire, or it works as an add on phone for an existing system. I know it works with other Panasonic DECT base units, may work with non Panasonic ones too.
All working, clean, batteries hold charge. Now unwanted as we are no longer having a phone line.