"As a single phone unit I do not have to 'register' that one do I??"
yes even single phone sets need to have the handset registered to the base . But check if the page button on the base works, if it does it's already registered.
See page 29 of the manual for how to register if it does need it
http://www.shop.bt.co...te_2100_userguide.pdf
I doubt it's a registration problem though, what happens if you swap the two phones over, that should tell you if it's the phone or the socket.