I have an English 4-phone Wireless telephone which has one connector to the wall socket.  I want to get an adaptor from BT (English) to the Spanish Telefonica wall socket.  The English end is known as a “BT” connector.  What is the uiversal name for the Spanish wall-socket?  Is a RJ45 or a RJ11?

It should be the same as the end that fits in the phone base unit which itself should be removable. you can then just buy a new cable with the same plug at each end, no need for an adapter.

RJ45 is for ethernet, RJ11 is the four wire standard phone connector.