[MovedGoalPosts]

First step is to contact ntl, who should be able to take action. If you have the evidence the person, if they used a private phone, may find themselves disconnected. Depends if they were that dumb.

If the person involved does it more than once, and it is evident, or should be evident to a reasonable person that theeir actions do or would cause distress, then there could be a strong case for harrassment. In that case ntl and your local police should work together as it's a criminal offence.