For the Nokia 3100 variants, problem with phone vibrating but not ringing, possibly with 'Silence' appearing in the RHS side of the window.
I've torn out most of what little hair I have left trying to cure this, but I've struck lucky on the web and the answer is ridiculously simple. Just unlock the keyboard and hold down the # key for a few seconds!
Doing the same thing again will put the phone back into vibrate only mode. If this is in the phone manuals I can't find it, but it works.
I figured out that I'd caused the problem when setting up an answerphone message, when I must have kept my finger on the # key a bit too long in one of the responses. But because I didn't know exactly when the phone stopped ringing I've only been able to associate the two things retrospectively.