Everything Everywhere announced that mobile customers of its two mobile brands, Orange and T-Mobile, can now access its two mobile networks as the first customer benefit of the merged company.
Orange customers can now make calls and send texts on the T-Mobile network and T-Mobile customers can do the same using the Orange network.
The firm said that customers who want to sign up for access to both networks will benefit free of charge, with no changes to their existing tariff or call or text charges.
Customers register at www.orange.co.uk/share or www.t-mobile.co.uk/share to receive an update to their SIM card.
From that point on, if a customer loses signal on their existing network, they will automatically pick up the signal from the other network where it's available.
Everything Everywhere said that it is launching a £4m joint Orange and T-Mobile advertising campaign on 11th October to let customers know the benefits from network coverage improvement.
Next year customers can expect benefits such as automatically switching to whichever of the two networks has the strongest signal while they're mid-call, and enhanced data and internet coverage.
Everything Everywhere CEO Tom Alexander said, 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," Alexander said.
"That means 27 million consumers can now keep close to the people, places and things that matter to them in more places than ever before."