Cable & Wireless Plc is stepping up the battle with British Telecommunications Plc with a new scheme that enables customers to use their BT phones to tap into the Mercury system: the Easy Access service is to start up next week in Reading, and in Gloucester a week later, and plans to extend it to its entire service area, which covers 80% of the UK population, within 18 months; existing British Telecom customers will be able to subscribe to Mercury some 48 hours after providing credit card and other details, and will access the Mercury system by prefixing outgoing call numbers with a three-digit code, 132, which flicks a switch at the exchange that sends the call through to the Mercury system; the new scheme is an alternative to customers having to buy a phone with a Mercury button used to prefix calls, which is seen as a barrier; to make the switch worthwhile, people need to be spending more than ?75 a quarter, much of it on long distance and international calls; the subscription fee will be ?11.75, which buys a saving of about 33% on off-peak long-distance calls, 20% on international calls.