Both consumer and business customers of Everything Everywhere's Orange and T-Mobile brands are now able to sign up to access the firm's two mobile networks.
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The move is the first outwardly tangible result of the Everything Everywhere joint venture,
As a result, registered Orange customers will be able to text and call over T-Mobile's network, and vice versa, should they lose signal on one network.
Further tweaks to this arrangement are expected in the coming months, including the capability to automatically switch between networks depending on which has the stronger signal strength, and enhanced Internet coverage - something providers have been under pressure to improve as smartphone uptake increases.
"Everything Everywhere's ambition is to provide all Orange and T-Mobile customers with the best network experience in the UK," said CEO Tom Alexander.
"Significant investment is taking place to ensure the network is enhanced for the 21st century customer," he added.
The JV between France Telecom-owned Orange and Deutsche Telekom-owned T-Mobile currently claims 27 million customers in the UK.
The switch on comes barely a week after Everything Everywhere announced it was
1,200 back office jobs in areas of duplication.