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To obtain ntl services legally you have to subscribe to ntl and they will hire the necessary cable modem to you as part of the monthly cost.
Originally some customers did own their own modems, but this is now the exception rather than the norm. Occasionally one of these genuine private modems are sold, but that will be rare. Most modems you see on sites such as ebay will not be the sellers property and cannot be legally connected to the cable network.
Cable Forum will not permit discussion of any methods that could allow customers to acquire services that they are not paying for.