Many thanks Russell. Good idea. I have attached a photograph of the the inside of the socket. I have done a load of searching and the only hint of anything similar came from an old B&O forum:
https://archivedforum.beoworld.org/forums/t/824.aspx
Tantalisingly close as I suspect an answer to the final question would have solved my question.
I have bared the wires but any tips about which to try with particular socket pins would be appreciated. Not sure if the phone is German or Danish or?? Done a lot of searching to see what wire codes they used in their lines but haven’t got anything which makes sense for this phone.
Many thanks again,
Kneale