Because the people manning the phones have to allot only a certain amount of time to each caller.Why they don't make notes for follow ups I have no idea but I suspect that with managment hanging over their shoulder they just can't really get down to sorting anything out and so things get 'forgotten' and along with that goes the idea that if it is important enough to you,that you will call again.However unless someone has a made a note you then have to begin all over again.I hope that you do get through to someone who can sort you out when you ring back.
Frankly as the person at the other end of the phone is NTL's frontline you would think more care would go into actually making time for not easy to solve problems at another NTL phone number which would deal with the less easy to resolve issues.
However how many of us get it right when we are asked to push a number for the service we require and how many just wait for a 'real' person to answer instead of actually trying to ring the correct section first?