Everything Everywhere has completed its signal sharing project, which means T-Mobile and Orange customers will automatically be switched between networks if signal strength is low.
Smartphones, tablets and dongles will switch networks if signal strength drops to one bar. If no 3G coverage is available, customers will be provided with 2G and switched back to 3G when it is in range.
The Smart Signal Share project has been part of Everything Everywhere 1.5bn upgrade plans.
There will be no extra charge for customers, and costs for calls, texts and internet usage will remain the same if users go beyond their allowance.
Olaf Swantee, CEO of Everything Everywhere, claimed the completion of the Signal Share project is another proofpoint to the firm's commitment to provide the best network in the UK.
"Smart Signal Sharing marks the completion of the Orange and T-Mobile signal integration project, which provides our 27.5 million customers with the widest 2G and 3G coverage available in Britain today."
"Our continued investment into the network, including the launch of 50 per cent faster 3G speeds through HSPA+, allows our customers to access the internet, social networks or download emails faster in more places than ever before."
Everything Everywhere is also planning to re-use parts of its 1800MHz spectrum to carry out a small-scale 4G roll out by the end of 2012.
Ofcom has yet to green light these plans to use 4G, and competing networks have unsurprisingly raised objections to Everything Everywhere's proposals.
"Ofcom's consultation on Everything Everywhere's application to vary its 1800 MHz spectrum licences to allow use of LTE and WiMAX technologies closed on 8 May 2012," the regulator said in a statement.
"The consultation responses raise a number of detailed issues that Ofcom must now consider carefully. We will publish a statement on the proposed variation as early as possible."
