Orange and T-Mobile customers can share both networks' signal to boost their chances of getting reception.
Apart from the obvious benefit of allowing 29 million consumers to use their phones more often, the sharing means there is less chance users will need to cancel their contract early, which could cost them (see the Cheap Mobiles guide).
The two networks, which merged in 2009, started to allow dual access on 11 October last year. All customers had to opt-in initially to use both signals.
But from the beginning of February, users started to receive text messages telling them they would automatically get access to reception from both.
If you're an Orange or T-Mobile customer you will only get automatic access to both networks once you receive the text.
The roll-out of texts will continue from now until the summer. You will pay the same charges as normal regardless which signal you use
Not received a text?
Those who have not received a message can still use both networks' signal.
However, they will need to register via the Orange or T-Mobile websites.
Simply enter your phone number and follow the prompts in the relevant link.
Many mobile users are forced to switch because, once their phone arrives, they find they cannot use it at home or at work.
Most providers give a window of between one and four weeks after purchase in which you can cancel. If you cancel outside that grace period you could be forced to pay the cost of line rental for the remainder of your contract.
See a network-by-network guide to cancellation rights but bear in mind that was printed last July so some firms may have changed policy.
Orange, for instance, is listed as giving no cancellation rights if you can't get signal but it now gives two weeks' grace for contracts bought in store or seven days for sales made over the internet or over the phone.
Those policies (in the link above) are voluntary if you buy from a store or when you sign up to a contract online or by phone that begins immediately.
If you buy a handset only (without a contract) via the internet or by phone, you have a legal right to return it and get a full refund within seven working days, for any reason.
If you sign up to a contract by phone or internet that begins within seven working days, your legal cancellation rights end when the service starts, unless the phone is faulty.
Further reading/Key links
Slash mobile costs: Cheap Mobiles
Sell old mobiles: Sell Old Mobiles
iPhone guide: Cheap iPhones