1899 and 18866 keep tripping over each other, and they feature often in discount forums. Both seem to offer incredible call deals - and seem to have survived for quite some time.
http://www.call1899.co.uk/voip.php - they also have a beta VoiIP service
http://www.call18866.co.uk/mobilerates.php - rates slightly different to 1899, but lower connection (1p instead of 3p)
I don't know if they are two different companies, or two brands from the same one?
No way to have prefix access on NTL though.