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From today, the newly-formed company, Everything Everywhere offers customers of Orange and T-Mobile the chance to sign-up to use their mobile phones to make calls and send texts across both mobile networks without additional charge to the user's tariff.
Tom Alexander, CEO of Everything Everywhere, said, "Today's switch-on is the culmination of a unique and hugely complex technical project. But the result is simple - our customers now get two networks for the price of one."
Significant network investment is taking place to ensure the network is enhanced, with customers being able to automatically switch to the network with the strongest signal mid-call from next year, said the companies.
The merged Everything Everywhere became the biggest mobile network operator in March this year with some 30 million subscribers.
Orange and T-Mobile also launched a £4m campaign today to advertise the services to its consumer and business customers.