A phone scam designed to trick people into thinking they had voted for contestants on ITV shows I'm A Celebrity ... Get Me Out Of Here! and Britain's Got Talent has been rumbled by investigators.

People ringing the fake phone lines thought they had successfully registered their approval for their favourite competitor when a voice at the other end of the line said: "Thanks for voting". But unbeknown to them they had mis-dialled and called an almost identical number designed to take advantage of people who got one digit wrong when making the call.

Calls to the numbers were charged at £1.02 plus standard network charges, and PhonepayPlus, the premium rate phone number regulator, said it was "a scam designed to generate illegitimate revenue".

The regulator investigated after receiving a complaint from a viewer who had attempted to call one of the Britain's Got Talent voting lines, 0902 044 2401, but had accidentally dialled 0905 044 2401 instead.

On getting through the viewer heard the "Thanks for voting" message, though it "did not give an indication of what the vote was for", the regulator said.

The investigators found that a Birmingham company, Subhan Universal Limited, was operating 10 phone numbers that were one digit different to voting lines used on Britain's Got Talent as well as I'm A Celebrity ... Get Me Out of Here!. The regulator monitored the service and found that all the numbers were active and rang through to the voice recording.

The number of calls made has not been disclosed, but the vast majority were made during the voting periods for the shows, and the calls peaked on the days leading up to the finals.

The regulator said it thought it was unlikely many people would complain "as consumers who interacted with the service were likely to have believed they had successfully voted for a contestant". The service operated between 6 May 2012 and 31 May 2013, when it was suspended.

PhonepayPlus imposed a formal reprimand and a £6,000 fine. It has also said the firm must fully refund within 28 days all those who were caught up in the scam who ask for a refund.

The regulator said the company had claimed it had allocated the numbers to another Birmingham firm called Telecom Media, which had planned to use the numbers "to run voting services on current issues", but it failed to provide adequate evidence that Telecom Media existed. The regulator said that "on the balance of probabilities" the service was operated by Subhan Universal Limited.

Separately, telecoms regulator Ofcom has announced a number of changes to phone charging, including one that means "Freephone will mean free". As a result, consumer calls to Freephone numbers (0800, 0808 and 116), which are generally free from landlines, will become free from mobile phones too, while the service charge for premium rate (09) numbers will be capped, "helping to protect consumers from the risk of rogue operators imposing extremely high charges in future".

Ofcom also said "confusion" around 0845 numbers would be addressed. This number range, which is sometimes tied to the cost of a geographic call, will no longer work in that way, and will instead function like any other 084, 087 or 09 numbers. That means the cost of calling 0845 will become clearer. All the changes will take effect in June 2015.