The only time you can dial in from a BT line is if you have an offnet account or a freedom account, then the number you are dialling form will need to be on NTL's IN platform for it to be recognised.
If I am away form home and have to use dialup I tend to use freewire, the dial up number is 0845 088 0088 and you just make up your own user name and password.
you can find them
here
HTH