In April, Vodafone introduced new international roaming charges for 60 countries outside the European Union, and told customers via text message.
Ofcom said it had “intervened” because it had “concerns about the clarity of that text message”.
In a statement, Vodafone said its customers “like” its products.
Vodafone’s Roam Further charges, introduced in 2017, mean customers on a monthly contract are charged a flat fee of £6 per day to use their phone in 60 countries including the US, Canada and Russia.
The changes mean customers will be charged the flat £6 fee even if they only send one text message.
Previously customers were charged for their usage and fees varied between countries.
Ofcom considered that light users would be more affected by the changes because “they could no longer send texts, make calls or use data on a ‘pay-per-use’ basis'”.
Mobile phone networks must give customers 30 days’ notice of any contract changes likely to be of “material detriment”, and must let customers end their contracts without penalty if they do not agree to the changes.
Some customers had complained and Ofcom said it was concerned that Vodafone’s communication had not been clear.
It said right to exit information should be provided “in the main body of the text or email” rather than linking to a separate webpage as Vodafone had done.
In response to Ofcom’s concerns, Vodafone sent another message to affected customers and gave them a further 30 days to leave their contract.
It said it had identified 55,000 customers who were likely to face “material detriment” following the introduction of the new charges.
In a statement, Vodafone said Roam Further “removes the fear of bill shock”.
“We took on board Ofcom’s views about notifications and will be sure to follow them,” it said.