The company has ended the registration process in the regions that it bought from Cable & Wireless last year. These include Greater Manchester and large parts of London and the south coast.
According to its website, the company is focusing on its broadband services and is only capable of offering dial-up internet access to a limited number of customers.
However, in a Daily Telegraph report, a spokesman gave a different explanation, saying that the company needed to focus on delivering NTLworld to the thousands of customers that have been waiting for more than a year for the service.
NTL has been keen to promote its high-speed broadband service, launching a £5m advertising campaign earlier this year. The service has about 30,000 subscribers, each paying £25 a month.