For which make of telephone do you require an instruction manual ?
(try the link(s) at the bottom of the page if you can't find your manual on the list)
The following link(s) have telephone and manualsavailable, some for free download:-
www.tabenterprises.co.uk is a website where you can get a manual for pretty much any mobile phone.
http://www.homephonesonline.co.uk/downloads/default.asp try that for BT panasonic and philips free downloads and also other makes