I would expect that at some point numbers would be reused at an address as there are only a finite number of available number combinations. I take it these are calls from people that used to know the previous owners of the number before you got given it? Sounds like someone moved without telling people that their number was changing

Not really NTLs fault if someone has done that but not much fun for you either.
If it's companies calling then you can get your number listed on the TPS list which will cut a lot of cold calling. If it's just people that still have the number in the address books for people they know then you are going to suffer it for a while. There is also the fact that it could just be that someone has a similar number to yours and someone is miss dialling. We get calls for a local school now and then as their number is only 1 digit different to ours

I was at work on Tuesday and in the space of an hour we got 3 "wrong number" calls and our number is an internal number for BT callcenters to use lol.
The odds are that you have been unlucky in receiving these calls but I can't think of much NTL can do to stop it from happening unless they can give you a number that they can guarantee has never been used before anywhere in the country for any area code region.