O2 is to hike the cost of texts and certain calls made outside your standard allowance from next month.
Customers who will be affected include pay monthly and SIM-only users who exceed their texts and calls limit, or use a service outside of their package.
The charge for a late bill payment is also set to increase as part of the changes, which come into force from August 2.
O2 hasn’t said how many people will be affected by the higher rates, but it has 36million customers in the UK.
Pay-as-you-go users won't notice any changes to their call and text prices.
The changes come after O2 also confirmed it is changing its monthly roaming limits for Brits travelling to 48 countries in Europe.
Customers will now be charged £3.50 for every gigabyte of data used above a 25Gb limit from August.

O2 price increases explained
Again, these charges will only affect pay monthly and SIM-only customers who exceed their limit or use services outside of their standard allowance.
From August 2, the following increases will come into force:
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UK text price will increase from 15p to 30p per message
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Charges to 0870, 0845, 09 and 118 numbers will increase from 55p to 65p per minute.
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Calls to 076 numbers will increase from 55p to 65p per minute.
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Cost of summary paper bill will increase from £1 to £2.
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Cost of itemised paper bill will increase from £1.50 to £2.50
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Late payment charge will increase from £3.50 to £6.
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Calls to 05 numbers will decrease from 55p to 35p per minute.
A spokesperson for O2 said: “After reviewing our out of bundle and additional services pricing, we’re writing to customers to advise that the cost of some optional extras is changing.
“Customers will only be impacted if they choose to use these services - their existing tariff, and any Bolt-On or personal allowance, remains the same.”
Not happy about the changes? Here’s what you can do
O2 says some customers who are still in contract will be given the right to cancel penalty-free as a result of the changes.
However, if you have a deal where you're also paying for your device, this part of your contract will need to be paid off in full before you can fully cut ties with O2.
It may also be worth calling O2 to see if they can offer you a better deal - for example, they may be able to move you onto a cheaper tariff.
You can speak to O2 by calling 0344 809 0202 or visiting its website.
Finally, if you're out of contract, you will be able to leave O2 without any charge and move to another network.
Use a comparison website such as Uswitch to find the best deals right now.
Remember to take into account how many calls and texts you actually use, as if you go for a package just because it is the cheapest, you may end up spending more on extra calls and texts on top of your package.
If you do find a great deal, but don't want leave O2, it is worth flagging the better price to O2 to see if they can match it.
While haggling is never guaranteed to work, it doesn't hurt to see if they can get you a better price.