Vodafone will soon charge its pay-as-you-go customers by the minute rather than the second - a move that will raise the cost of most calls.
It means a call lasting one minute and one second will now be charged as two minutes - costing 50p instead of 25p.
The phone network said the change - which comes into force from 1 August - would "simplify" charges for its customers.
Other networks, such as T-Mobile and Orange, also apply this billing method.
Vodafone sent text messages to its pay-as-you-go customers notifying them of the change.
"No more complicated pricing by the second," read a statement on the Vodafone website. "Just clear, straightforward per-minute charges.
"So if your call is a minute and 37 seconds long, it'll be charged as two minutes. That way, you'll always know exactly how many minutes you have left."
Some customers described the message as "patronising".
Posting on Vodafone's support forum, user Funboy said: "This is being introduced to make more money for [Vodafone] pure and simple.
"It doesn't make anything simpler other than the fact that PAYG customers will essentially pay more for calls."
Calls for pay-as-you-go customers on Vodafone currently cost 25p per minute - with a minimum of one minute's charge for each call.
Telecoms expert Adam Kirby from uSwitch.com added to the criticism.
"While simpler and clearer pricing is always welcomed, it shouldn't come with the sting in the tail of increased bills," he said.
"But the real gripe is the way the message was communicated. Sending a text may be a quick, simple and cheap way to communicate with your friends, family and even your boss, but Vodafone breaking the news about price changes this way may be a bit of a stretch.
"The short and sharp nature of a text may explain why the message failed to tell the whole story - and so went down like a lead balloon with some customers."
Vodafone has defended the change, saying in a statement: "We believe that by offering propositions with a generous allowance of minutes, we continue to offer our customers great value.
"Many of our competitors already offer price plans charged in this way."
Customers with Orange or T-Mobile are already charged on a per-minute basis.
However, O2 and Virgin Mobile still have a per-second model, with a one-minute minimum charge.