Unauthorized data sent over 3G networks on Windows Phone 7 devices was caused by a third-party application, Microsoft has said.

Microsoft said last week that it was investigating the problem after Windows Phone 7 users complained that their devices were arbitrarily sending out data for no reason. These phones allegedly sent out the phantom data over the 3G network, even when the phone was connected to Wi-Fi. One user reported that his phone was sending out 30-50GB of data inexplicably every day. This issue would cause inflated cell phone bills, although Microsoft did not address any kind of compensation for those affected.

Microsoft said it is continuing to investigate what caused this hiccup in the operating system.

"We have determined that a third-party solution commonly accessed from Windows Phones is configured in a manner that potentially causes larger than expected data downloads," said a Microsoft spokesperson. "We are in contact with the third party to assist them in making the necessary fixes, and are also pursuing the potential workarounds to address the configuration issue in case those are needed. At this point in our investigation, we believe this is responsible for most of the reported incidents."

Although Microsoft did not disclose which program caused the problem, complainants widely believe it was an email application. The company also said that the issue only plagued "a small (low single-digit) percentage" of Windows Phone 7 users.

Windows Phone 7 is still a pretty new operating system. The first round of devices went on sale in early November, and Microsoft will continue to add more phones to the fold this year.