Mobile phone customers with Three will be able to call 0800 numbers for free from today.
The operator said people signing up to their pay-monthly tariffs will be able to make 0800, 0808 and 0500 calls - which costs Brits around £600 million a year – for free.
And existing customers can move across to the new tariffs without incurring any fees or having to extend their contracts so they too can benefit from the change in policy.
Three has also cut the cost of calling other 08 numbers, such as 084 and 087, which typically cost between 14p and 40p per-minute to just 5p per minute.
The changes are in response to research conducted for Three that found millions of customers had no idea how much it costs to call freephone numbers. It found 75% of mobile phone users have no idea how much their operator charges them for calling 0800 numbers and 10% think they are free, as they would be from a landline. Nearly half (47%) have also experienced an unwelcome shock when opening their bill due to premium numbers.
Chief executive Dave Dyson said: "We asked people what frustrated them most about their mobile bills and confusion around the cost of calling 08 numbers came out top by a big margin. These charges are often buried in the small print and they're expensive enough to make most customers fear calling them from their mobiles.
"We want UK mobile users to get the most enjoyment from their smartphones or data service - the cost of calling 08 numbers is something we want to address for our customers."
Ernest Doku, telecoms expert at Uswitch.com said the move should be welcomed.
"Considering it costs nothing to call a 0800 number from a landline, charges for calling freephone numbers from mobile phones should have been outlawed years ago," he said.
He added that the Three tariffs should help keep monthly bills down. "According to uSwitch research, almost half (48%) of pay-monthly mobile customers pay extra on top of their set tariffs every month, with more than a quarter (26%) of these customers overspending due to calling premium rate numbers.
"If a number is free, then it should be free wherever you call from. By enabling consumers to set self-imposed limits on calling these costly numbers, Three is giving mobile users greater - and much-needed - protection from higher than expected bills."
In December, telecoms watchdog Ofcom announced freephone numbers (0800, 0808 and 116) will be free to call from all landlines from June 2015.