This refers UK old style converted dial phones.
Sounds like the springs have weakend or dirt has built up around the line on/off bar. From description is it the old dial type ? if so one screw at back hold case on undo screw rais back of case easing it over dial. clean up around bar and check springing while case off. If still week you have to find new ones and replace them don't know how but you should see.
clean case as well. If plug in remove plug from line socket before removing back.
nobby
January 2009