Almost 800 call centre staff in Swansea are having their employment contracts transferred to a different company, but none will lose their jobs.
As part of its plans to outsource or cut 6,000 jobs UK-wide, NTL said its workers in the city would move to IBM.
It said the changes affecting its 791 staff in Swansea would take effect from July without redundancies.
The changes come as NTL completes its merger with Telewest and looks to achieve cost savings of £250m per year.
Chief operating officer Neil Berkett said: "We recognise that today's announcement will mean uncertainty for some employees and the communities they live in.
"We will continue to communicate as honestly and openly as possible with our employees throughout the process and provide whatever support and advice is needed."
Welsh Assembly Government enterprise minister Andrew Davies said he had been assured none of the Swansea employees would lose their jobs.
"This is very good news for Swansea," he said.
"Whilst the uncertainty of today's announcement will be worrying for other parts of the UK, the decision to retain employment at Matrix Court reflects both the quality of the people who work at the Swansea operation and the outstanding reputation of the Welsh contact centre industry."