A string of mobile phone providers have been hiking their charges for calls to numbers starting 084, 087, 09 or 118, a comparison website has found.
USwitch.com said that the spate of increases means that consumers could, for example, find themselves paying more to vote on their favourite TV shows.
The rises follow changes introduced in July by Ofcom, which aimed to make it easier for consumers to know how much it would cost to call a non-geographic or premium rate number.
The cost of calling such numbers is now split into two parts - an access charge and a service charge.
The access charge goes to a consumer's mobile phone company, charged as pence per minute. This should be made clear on bills and when someone takes out a contract.
Meanwhile, the service charge makes up the rest of the call charge. The organisation that the consumer is calling will decide this, and will tell the consumer how much it is.
USwitch found that some providers have nearly doubled their access charges recently. It said that Three, which contacted customers this week, will impose an 80% increase on its charge from November 3, meaning the access charge will jump from 25p to 45p.
The changes mean that, for example, if a Three customer calls the Strictly Come Dancing vote line from November 3, the call could now be 60p per minute, instead of 40p previously, uSwitch said.
It said that O2 and Vodafone had already imposed similar increases in August, also taking the access charge to 45p.
Ernest Doku, telecoms expert at uSwitch.com, said: "Only this week, Three announced it is increasing its access charge costs from 25p to 45p. That's before you add on a service charge for the number you want to call, which can be anything up to 13p per minute for numbers starting with 087, for example.
"Ofcom's new system relies on people being vigilant and keeping an eagle eye on what they will be charged before they pick up the phone. The charges are meant to be more transparent than before, but with significant price hikes since the new system was introduced, consumers need to be savvy to make sure they are not left out of pocket."
Mr Doku said that it is worth consumers looking closely at what numbers their mobile operator includes within their tariff.
He said: "Some providers, such as Virgin Mobile, include calls to 084 and 087 numbers within a number of its Pay Monthly packages, which means you will not be charged its 36p access charge for calling those numbers."
A Three spokesman said the new access charge was being introduced following a review of its pricing.
He said: "There are now many alternatives to calling expensive numbers. Of the 100 most dialled 084 or 087 numbers on Three's network, 67 have free or lower cost alternative numbers customers can call.
"We are committed to ensuring the cost of calling 084 and 087 numbers is clear for our customers and that we support customers in finding alternatives to using these numbers."
A spokeswoman for Ofcom said the changes introduced on July 1 have made call charges clearer, with callers being able to see what they are paying and where their money is going.
She continued: "But some prices have changed, and so we encourage people to shop around for a tariff that's right for them.
"Phone providers are now offering a range of pricing options for these numbers, including per-minute access charges as low as 1p per minute.
"Ofcom actively encourages organisations to use 03 numbers, which we set up to cost no more than a normal 01 or 02 number. They must also be included in a customer's inclusive minutes and discount schemes in the same way."