Data roaming is the phantom menace
Originally posted by 5corpio
Disabling internet access when abroad should save you money, but Emma Lunn faced an unexpectedly high bill on her return. 02 says the problem is fixed – but is it?

Think that switching data roaming to "off" on your iPhone while abroad will protect you from expensive overseas data bills? Well, think again. Some O2 users have experienced "phantom roaming" charges, despite turning off the data roaming function before they leave the UK.

When they return to a shock bill and complain to O2, they meet with a range of responses: some are refunded the charges, others have their request for a refund turned down.
O2 admits there was "talk of a software issue" last year, but says this was resolved several months ago and insists there is no longer a problem. However, a handful of recent cases raise questions about whether this is true.

Data roaming is the term used when you use another mobile network to access the internet on your phone while still being billed by your normal provider. It is expensive and not something covered by any data allowance on your contract, which is for the UK only.
For this reason, mobile networks advise people to turn data roaming off while abroad. On an iPhone this can be done by going to Settings and selecting General and then Network. You can then switch data roaming on and off. The iPhone can still be used to access local Wi-Fi networks, which are either free or billed directly (not via your mobile network).

During a trip to Mexico in January, I made sure I switched off data roaming on my iPhone before I left the UK. However, I returned home to a bill showing £40 of internet and data costs while abroad. I contacted O2, which carried out a "network investigation". This supposedly proved the costs were genuine. O2 customer services suggested it was a fault with my iPhone – provided by O2 less than a year earlier – and recommended I visit an Apple store to have it checked.

I was too busy and decided to write off the £40. But that was before realising I wasn't alone. A Google search found a number of forum threads – on O2's site, apple.com and moneysavingexpert.com – where O2 users believed they had also been billed for phantom roaming
Link: Data roaming is the phantom menace | Money | The Guardian