BT telephone manuals can be downloaded at http://www.bt.com/customerservices/userguides.jsp?parentcat=cs_userguide
If you can't find the manual at this site then :-
BT have introduced a charge for their instruction manuals which were previously free to customers. The charge is normally £5. To obtain one you will need to ring the company BT now use to supply manuals on 0800 980 8999. You might however try ringing customer services on 150 to try to get a free one if you are a BT customer but you probably won't have much luck.
Any manuals directly available from this site are listed below.