21CN is a secure and intelligent communications infrastructure designed to deliver existing and future services, according to BT.
Phase one of the upgrade, due between November 2006 and March 2007, will see the upgrade of voice services to some 10 per cent of customer lines in Cardiff and the surrounding area.
Phase two, from April to mid-May 2007, will deliver a further 10 per cent of upgraded lines.
By the end of Phase 3 in the summer of 2007, BT will have upgraded all 350,000 customer lines, 90,000 of which also support broadband and ISDN2 and ISDN30 services.
"Years of planning, testing and development will culminate in South Wales in three months when we start the exciting process of bringing the world's most advanced national communications infrastructure to everyone the UK," said Paul Reynolds, chief executive of BT Wholesale.
At the end of the final stage, BT and other communications providers will begin an in-depth review before moving to the planned national upgrade of all remaining customers across the UK - some 30 million lines in over 5,500 telephone exchanges - from January 2008.
BT explained that the upgrades occur within the exchanges and that customers will not have to do anything. Telephone numbers will not change and roads will not be dug up.
The telco has also announced a single communications programme designed to inform all consumers and single-site small businesses of the upgrades, regardless of their service provider.
Further details of this programme will be announced in October.
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