Mobile phone users are reportedly being hit with huge bills because their phones are switching from free wifi to expensive 3G without warning.
Many people take advantage of their home broadband connection or the free wifi offered by hotels or cafes to surf the internet or download music and video without eating into their monthly data allowance.
But, according to the Daily Mail, many people who do this are still being stung with high bills because when the wifi connection fades or cuts out, they're being automatically switched to 3G instead.
The paper cites the case of 57-year-old Pat Ingram, a retired IT specialist from Poole, who was astonished to be told she'd used up her entire monthly mobile data allowance within a week while using her new iPhone 6.
"EE messaged to say I was close to my limit, but it was a shock as I had only used data when I thought I was on wifi," she said.
Worse, there's the case of the 13-year-old Vodafone customer who clocked up 600MB of usage in six days - more than she usually used in a month - while she thought she was using her home wifi.
Last October, the Mirror reported on a woman who had a similar experience while on holiday in Bodrum, Turkey. She used - or so she thought - the hotel wifi to download two episodes of My Family. But the hotel wifi cut out, and her phone took over the download, landing her with a £10,611 bill.
Data roaming can be turned off through the settings menu on most mobile phones, while still allowing calls to be made and received. It's still possible to use wifi once this has been done, with no danger of an unexpected switch to 3G. Ofcom has video guides on its website showing how data roaming can be turned off on different types of phone.
And when using wifi, it warns, "Remember to stay within range of the wifi hotspot to avoid your device finding another network connection. Check to ensure the wifi icon is visible on your phone."
Even when users don't want to turn data roaming off, the phone companies will allow the customer to cap their monthly spend at a certain amount at home or abroad, making it impossible to go over the limit - customers should contact their phone company for more information.
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