An Apple product fan was shocked to discover that on the day he started using his new iPhone 4 he ran up a 300-yuan (US$45) bill within an hour - while he was sleeping.

When photographer Cao Zichen checked his account, he discovered this charge was for Internet use. The bill said he had used his iPhone 4 to connect to the Internet at 2 am, generating a network data flow for 58 minutes.

Cao said he was asleep at the time, although he didn't turn off his phone. "I found the data traffic was generated by some automatic download functions on my new phone," said Cao.

Cao's experience is not an isolated incident, as many owners of the popular high-tech product have complained online about their smartphones connecting automatically to the Internet and the "shocking and painful" experience of the subsequent bill.

An Apple support engineer with China Unicom confirmed to Shanghai Daily yesterday that the company had received a large number of similar complaints from iPhone 4 users.

Some web users speculated that the "phantom cost" of the data traffic could be a trap set by the phone and its operator.

"Neither China Unicom nor Apple Inc is 'stealing' money from users by consuming unnecessary data traffic," insisted the engineer, surnamed Zheng, with China Unicom's iPhone Technology Center.

"The cost incurred for network data traffic is caused by inappropriate settings on users' iPhones," he said.

Zheng said some iPhone 4 users believed they turned off data traffic-consuming applications (apps) by simply tapping the "Home" button, as was the case with the iPhone 3.

However, on the newer model those apps still run in the background.

The confusion is caused by iPhone 4's new multi-tasking feature, which allows users to run more than one app and switch through different apps when needed.

The new function means a single tap on the Home button will no longer turn off the apps, but only return to the homepage. To turn apps off, users must double-click the Home button, press the apps icon for about two seconds and then press the "Quit" button.

iPhone 4 users may not be aware of this and end up with big Internet use bills, Zheng said.

Other settings can also leave users with higher bills for Internet connection fees.

Zheng recommended that iPhone 4 users turn off the "Push" and "Notification" functions on their cell phones when the phones are idle, as these functions allow their phones to exchange data rapidly with e-mail servers and with other apps.

However, Zheng didn't say why users were not given these tips when buying their phones.

"Sellers should be obliged to make this clear to buyers so that we can avoid the inconvenience and high charges," said Cao.

For technical help with iPhone 4, China Unicom customers can call 400-677-8899.