Customers will need to sign up for the feature, which is free to use and will enable a handset to automatically swap networks if they lose signal with their original network.
The service will be made available from October 5. However, it will only apply to 2G services so while those calling and texting will be able to use the function, it'll make little difference to those surfing the web from their handsets.
Orange and T-Mobile first announced plans to merge last year and were given the go-ahead by the European Competition Commission in March. The parent company created by the merger is known as Everything, Everywhere.
Everything, Everywhere said by next year handsets will automatically switch between the two networks, without Brits needing to sign-up to the service. The company also said it will roll out the service to its 3G networks in the future. As part of this Orange customers will be given the chance to use the Mobile Broadband Network Limited (MBNL) 3G network, which is a joint venture between T-Mobile and 3.
"This literally takes it away overnight."